The single lens consists of a one piece of lens which is provided with a spherical surface and a flat surface on either side or spherical surfaces on both sides and by means of which an incoming light is so deflected that it will be emanated or focused onto one spot.

There are two kinds of surfaces in a spherical surface (convex and concave surfaces), whereas lenses include a plano convex lens equipped with flat and convex surfaces, a plano concave lens having flat and concave surfaces, a double convex lens consisting of convex surfaces on both sides, and a double concave lens made up of concave surfaces on both sides.

The plano convex and concave lenses are used for producing an image (zooming in and out an image) and collecting a beam of light, which are selectively used depending on their use conditions.

The plano convex lens should normally be so disposed as to direct its convex surface toward an object and to locate its flat surface on the side of an image.If an object is located at an infinite distance (parallel light), the convex surface should be directed toward the object and the flat surface be located on the side onto which a beam of light is focused.
For the double convex lens, there is no problem with its direction because the convex surfaces have the identical curvature radius.

The plano concave lens and double concave lens are used to emanate a parallel light beam, both of which are used separately depending on their use conditions.

When they are used for producing an image (zooming in/out an image), the image usually becomes a virtual image, and thus they become used more often in combination with each other than being used independently.

The plano concave lens should normally be so disposed as to direct its concave surface toward the side of incoming light (parallel light beam) and to locate its flat surface on the side of outgoing light.However, for the high-output laser beam, the flat surface should be located on the side of incoming light, so that no focus will be placed on the concave surface which is irradiated with the incoming light.
For the double concave lens, there is no problem with its direction because the concave surfaces have the identical curvature radius.

The ball lens whose curvature radius is especially smaller than other spherical lenses is primarily used for focusing a beam of light onto optical fibers and light coupling, or collimating an outgoing light from the fibers. Because its curvature radius is the smallest in effect, the spherical aberration becomes larger as compared with that of ordinary single lenses, and as such this lens is not generally suitable for imaging applications.

When a laser beam is guided to an optical fiber, a ball lens which is to be selected should be identified based on the fiber's numerical aperture (NA) and the beam's diameter. The ball lens's NA to be selected should be smaller than the optical fiber's NA.If a ball lens whose NA is larger should be selected, part of amount of the beam which entered the optical fiber through the lens will not propagate while total internal reflection takes place inside the core of fiber. As a result, the light eventually will lead to light transmission opening. For your information, the ball lens's NA can be determined by the following expression:

Because the curvature radius of a ball lens is very small in comparison with that of ordinary single lenses, an obtainable NA will increase sharply (see Fig.3) if the ratio (d/D) of an incoming beam diameter (d) to the lens's outer diameter (D) becomes larger (see Fig.1).This is nothing less than the lens's spherical aberration.

As an example of introduction of the aforementioned expression, let us consider a fiber coupling of laser beam here.The NA of the CorningRSMF-28TM, which is one of the standard fibers, is 0.14 at the wavelength of 1,310 nm.Then, if the 1310-nm semiconductor laser beam with a beam diameter (d) of φ1.㎜ is guided to this fiber, n will equal 1.50 for the ball lens of N-BK7 at the same wavelength; and thus D = 8.09 mm will hold based on the aforementioned expression.

Basically, a ball lens should be so disposed as to bring it into almost contact with fiber edge.Incidentally, although it is possible to calculate a ball lens's effective focal length (f) and back focal length (B.f) which can be determined by the following formula, please especially note that because it will vary depending on d/D, the formula should be used only as a guide.

In the case of light propagation application (fiber coupling application) between the fibers having the same NA, two ball lenses should be used (see Fig.2).
In this case, the area between the ball lenses should be basically situated under the condition that it remains kept contact with air.

First surface mirror and second surface mirror

Almost all the mirrors used for optical purpose generally adopt the design of a first surface mirror wherein a metal coating having high reflectance properties or a dielectric multilayer (hereinafter called enhanced reflectance coating) is vapor-deposited on a substrate (glass or metal).(See Fig.1 for details.)Unlike a first surface mirror, a second surface mirror (see Fig.2) which is usually used for our daily lives is covered with a protection plate glass or plastic which is overlaid on the enhanced reflectance coating (a silver coating in general) which is vapor-deposited on a substrate.This approach makes it possible to prevent the silver coating from being scratched by mistake, and the same coating which easily gets oxidized from being exposed to air.For the first surface mirror used for optical applications, it should be handled with utmost care because it may be often scratched accidentally on the ground that its enhanced reflectance coating is exposed to the outer surface. In addition, the reasons that no second surface mirror is used for any mirrors for optical applications are as follows:

● At the time of oblique light incidence, the incoming light is refracted by a protection plate glass. For this reason, a light path of reflected light is shifted because of the enhanced reflection coating.
（Red solid line in Fig.2)

● In addition to the reflection from the enhanced reflection coating, a surface reflection (Fresnel reflection) of 4% or so occurs on the medium boundary surface between the protection glass plate and air (as shown by the red broken line in Fig.2).This Fresnel reflection causes such an adverse consequence that a reflected light (image) will be doubly projected.

● Because of the above described Fresnel reflection, the index of refraction of enhanced reflectance coating will decrease.

In an effort to meet the diverse needs on the market, we offer several metal-film coatings for mirror as standard coatings which are tailored to actual wavelengths to be used.

Generally speaking, a metal film is a very delicate coating of which mechanical strength is insufficient. For this reason, overlaying a protection film, such as a dielectric material, on a metal film results in not only enhanced its handling ability but also improved easy-to-clean performance in itself. Furthermore, because a such measure prevents the metal film itself from being oxidized and improves the durability, the protection film has become an essential element recently. All the optical mirror products for optical applications that have employed the metal films which are introduced in this catalog are provided with this protection film.
When the mirror surface is heavily soiled, either isopropyl alcohol or the acetone solution can be used to wipe it clean manually.

Before cleaning a metal film, please refer to the following procedures:

● Use an air cleaner or the like to blow off dirt and dust which are deposited on the surface of metal film.
● Basically, perform a manual wiping work using a cleaning fluid only when fingerprints are put on the metal film surface or
the surface is heavily stained.
● As a cleaning fluid, be sure to use such an organic solvent as the isopropyl alcohol or the acetone solution.
　With a lens paper moistened with a small amount of solvent, wipe and clean dirt deposited on the metal film surface off
slowly and gently.
● Care should be taken to see that the entire cleaning fluid has been removed or it has not been wiped off unevenly.

It should be remembered, however, that optical components as exemplified by mirrors are highly vulnerable to high humidity. If a metal film is left as it is in a highly humid place, mold develops immediately on it.
If it has to be stored in a highly humid place for unavoidable reasons, be sure to use a drying agent, including the Silica GelⓇ, etc.

1. Without coating (Uncoated):
It is a glass substrate (mirror finish) which is not covered with a metal film or a protection film. It is best suited for a substrate which requires a special mirror coating.


The protected aluminum coating (Protected Aluminum) is the most typical metal coating which is used for wavelengths between the visible and the near infrared.As a protection film, the silicon monoxide (SiO) of 1/2 wavelength film is overlaid on this coating. The average index of refraction is 85% or higher in the range from 0.4 nm to 0.7 nm.

3. Enhanced reflection aluminum coating (Enhanced Aluminum):
It makes improvements in the average index of refraction of visible light in the range from 0.45 nm to 0.65 nm to increase it to 95% or more by protecting the aluminum coating with a proper dielectric multilayer.
Although this coating is more expensive than the aluminum coating, this coating is recommendable if higher index of refraction is required in the visible range.

4. UV enhanced aluminum coating (UV Enhanced Aluminum):
It provides greater improvement in index of refraction in the ultraviolet ray band by protecting the aluminum coating with a proper dielectric multilayer.In the range from 0.25 nm to 0.7 nm, this coating can achieve the average index of refraction of 85% or higher.

5. Gold coating (Protected Gold):
It works very effectively when higher index of refraction is desired in the near infrared ray and infrared ray regions.The average index of refraction of 94% or more and 97% or more can be attained in the range from 0.7 nm to 0.8 nm and from 0.8 nm to 2 nm, respectively.As is the case with the aluminum coating, the silicon monoxide (SiO) is used as a protection film.

6. Silver coating (Protected Silver):
It works very effectively when higher index of refraction is desired in the visible range and infrared ray region. The index of refraction of 98% or more can be attained in the range from 500 nm to 800 nm. Although the characteristics of the silver which easily tarnishes can be alleviated through processing of a protection film, it is advisable to use it in a low-humidity environment.

The mirrors having a high-precision polished finish which are used for interference experiments making use of laser's coherency and astronomical researches to observe stars at a great distance are generally provided with low surface roughness of their substrate prior to application of reflection coating, and most of such products typically have the surface roughness of 1/8 to 1/10λ or below. For a mirror, the wave surface of a beam of light reflected from the mirror depends on the surface roughness of a substrate on the reflection surface. In this case, as the light path is returned again after the light is shed on the reflection surface (in the case of vertical incidence), a size of the reflection wave surface virtually becomes twice as much as that of the surface roughness. By way of example, the reflection wave surface is 1/5λ for a mirror that has the surface roughness of 1/10λ.Giving consideration to these facts follows that the substrate surface roughness from which the wave aberration (Rayleigh limit) of 1/4λ or less which can be regarded as non-aberration can be obtained should be limited to those mirror products that have the surface roughness of 1/8λ or less.

In contrast to the mirrors having high-precision polished finish, such mirrors (surface roughness: 4 to 6λ/25.4 mm level) that adopt a float glass substrate using a soda lime glass (blue plate glass) are widely used as reflection mirrors for image information (polychromatic light) applications. Because they are manufactured by float method, substrates which are inexpensive and relatively large in size can also be fabricated. Also, the mirrors (surface roughness: 1/4 to 1λ level) having standard polished finish which belong to a middle-class category between the mirrors having high-precision polished finish and float-glass-substrate-based mirrors employ a polished glass using an optical glass, such as BK7, as their substrate. In the event that those mirrors that have an intermediate property (performance) between the aforesaid two kinds of mirrors and adopt a float glass substrate are not sufficient in terms of practical use, the previous mirrors are available as the second option. These mirrors are also excellent in cost performance. Choose your favorite mirrors according to your budget and intended use.

In order to reduce the phase change quantity (phase shift quantity) of reflection wave surface due to expansion to a minimum, it is especially important to use a substrate with a low coefficient of thermal expansion. For the N-BK7 which is a typical optical glass used for mirror's substrate, its thermal expansion coefficient is 7.1 x 10^-6/℃, whereas in the case of Pyrex, it is below half that of the BK7 which is 3.2 x 10^-6/℃.For the synthetic quart, it is even lower than that, which is 0.55 x 10^-6/℃; about 1/6 of that of Pyrex.In the case of Zerodur, it is much lower than the former, which is 0.05 x 10^-6/℃; about 1/10 or lower than that of the synthetic quart.

As an example of phase change due to a difference in thermal expansion coefficient, comparison is made here between Pyrex and the synthetic quart, as mentioned below. For the synthetic quart whose substrate's wall thickness is 10 mm, because its thermal expansion coefficient is 0.55 x 10^-6/℃ as mentioned above, it has in effect the thermal expansion of 5.5 nm/℃.If by any chance such a temperature rise as 10 ℃ occurs, there is a thermal expansion which is equivalent to 55 nm (= 1/10λ [＠550 nm]）.In the case of mirror, as the light path is returned again after the light is shed on the reflection surface (in the case of vertical incidence), it is certain that the phase shift quantity which is twice that of thermal expansion volume actually will take place and the phase shift which is equal to 1/5λ will occur. In contrast, for Pyrex, the phase shift which equals 1λ or more will occur based on the same concept. As stated above, using the mirrors that employ the synthetic quart and Zerodur whose thermal expansion coefficient is extremely low as their substrate is of very importance for the application of aerospace engineering and other applications to require accurate coherence experiments.

A beam splitter (hereinafter called "B/S") is an optical component which divides an incoming light beam into two light beams by a predetermined specified split ratio. Reversibly, it can also be applied to the overlaying of two pieces of light. As typical shapes of B/S, there are the cube and plate types as discussed below.

The cube B/S (see Fig.1) consists of two prisms (usually right angle prisms).A cubic shape is formed by vapor-depositing a proper optical thin film to function as B/S onto the slanting surface of the other prism and putting together both slanting surfaces of two prisms. The cube B/S has the following advantages which a plate B/S (as described later) equipped with similar optical functions does not have: No transmitted light is refracted; the light path length (Optical Path Length) of divided reflected light and transmitted light is the same; deterioration of a thin film itself occurs less frequently because the film to function as B/S is located inside a glass substrate and is not exposed to air; downsizing of the entire optical system can be expected because the light path length through the inner area of B/S is longer; its alignment work is eased because 45-degree incidence as in the plate type is not required owing to the fact that its incident angle condition is vertical incidence, etc. Conversely, its disadvantages are that it is heavy because of its cubic configuration, manufacturable size is limited, it is generally expensive, and so on.

Of the two right angle prisms used for a cube B/S, a proper mark is put on the sand surface of the other prism. On the slanting surface of the prism on which the mark is put, a thin film to function as B/S is overlaid. When using the cube B/S, be sure to use the prism of which slanting surface is marked as the incident side. Although it is no problem for it to be used in a reverse way, the durability of the bonded resin may be affected in such case.

The plate B/S (see Fig.2) is the beam splitter where a proper optical thin film to function as B/S is vapor-deposited on a thin plate glass. The plate B/S has the following advantages which the cube B/S equipped with similar optical functions does not have: It is light in weight because a thin plate glass is used as its substrate; a relatively large size can be manufactured; it is inexpensive and so forth. Conversely, its disadvantages are that the light path length of reflected light differs from that of transmitted light; the transmitted light is refracted; degradation of its optical characteristics is liable to occur because a thin film itself is oxidized owing to the fact that the film is vapor-deposited on the substrate surface; its alignment work is complicated because 45-degree incidence is required and so on. An amount of travel ("d" in Fig.2) of the incident light when it moves in parallel transmits by the action of light refraction can be determined by the approximate expression shown in the figure for the B/S product whose incident angle is 45 degrees and index of refraction of the substrate is 1.5.

Also note that the back side of substrate of some plate B/Ss is covered with an anti-reflection coating. Selecting the B/S having the anti-reflection coating is especially effective in the case of putting the B/S into an imaging optical system. In this manner, an unnecessary return reflection (Fresnel reflection) which occurs in a boundary surface between the reverse side of B/S and air can be eliminated, and B/S as optical system can be improved.

Standard type (hereinafter called the "Unpolarizing type"): Non-polarizing type and polarizing type

The B/Ss of unpolarizing type (Unpolarizing Beamsplitters) have been designed in such a way that an obtainable split ratio of transmitted light to reflected light is not classified by the type of polarization components of p and s components, while only its average is taken into account. Accordingly, no control is exercised as to how each of the transmitted light and reflected light is turned into p or s component to check what type of light intensity such component has. For this reason, at this stage, the design is based on the assumption that the incident light is the unpolarized light as typified by natural light. Consequently, when a randomly- polarized light beam whose polarization direction and polarization ratio change with time is entered, variations in transmitted/reflected light quantity occur as time advances. Moreover, when a polarized light beam as typified by a laser beam is entered, what often occurs is that the split ratio does not become 1:1, and a totally different result is produced. As mentioned above, the unpolarizing beam splitters are used for polychromatic light, and they are applied widely to such an application in which split ratio of p or s component needs not be considered separately, as 1:1 split of image information.

For the non-polarizing beam splitters, transmitted light and reflected light are split by a predetermined ratio, including polarization state of an incoming light beam. In the case of a non-polarizing B/S of which split ratio is 1:1, all of the p component (Tp) and s component (Ts) of a transmitted light to be obtained, and the p component (Rp) and s component (Rs) of a reflected light are divided by the same ratio. For this reason, this type of beam splitter is most suitable for 1:1 beam split of a polarized light beam as typified by a laser beam. Each polarization state of the transmitted light and reflected light which is obtained by a non-polarizing B/S will be the same as that of an incoming light ray.

For the polarizing B/S (beam splitters), a randomly-polarized light is split into a reflected light beam of s polarization and a transmitted light beam of p polarization separately. Accordingly, each light that is split as such will be placed in the polarization state. The beam splitter of this type is used for 1:1 beam split application of a randomly-polarized laser beam or the filtering application to obtain a linearly-polarized light ray which has a high quenching capacity.

For each type of beam splitter, please use the collimated light or an incoming light beam which is similar to the former in order to obtain the specified optical characteristics. Avoid using a divergent beam and a convergent light ray. Otherwise, it results in an aberration (especially for B/S provided with a dielectric film).

Functional differences of right angle prisms due to presence or absence of coating

For a right angle prism which is most frequently used as a 90-degree deflection angle prism, two types of prisms are selectable; one of them is a prism in which the prism's slanting surface is covered with aluminum coating (hereinafter called the "Protected Aluminum") and another one is a prism which is not covered with such coating (hereinafter called the "Uncoated").When a maximum throughput needs to be obtained from a prism, it is the best choice to make use of "total reflection (Total Internal Reflection)" phenomenon of a beam of light using uncoated prism. However, in order to realize 100% reflection on the prism's slanting surface through total reflection, it is required that a parallel light beam in a state close to vertical incidence (0-degree incidence) be entered with respect to the prism entrance plane. As can be seen from the spectral reflectance characteristics, 100% reflection cannot be expected from the Protected Aluminum because a little light absorption by the aluminum coating itself occurs. On this account, this aluminum coating should be used for the purpose of further enhancing the index of refraction with respect to the light incidence which is not vertical incidence (under the use condition that no total reflection takes place on the prism's slanting surface).

Functional differences of right angle prisms due to differences in glass

In our optical glass product lineup, three types of products are available as right angle prisms; N-BK7, high-quality synthetic quart and N-SF11. While the N-BK7 (nd = 1.517) is the most standard optical glass among them, the range of incident angles in which total reflection on the aforesaid prism slanting surface can be achieved is able to be increased by fabricating a prism using the N-SF11 (nd = 1.785) which is a glass with a high refractive index. The high-quality synthetic quart (nd = 1.458) excels in transmissivity to ultraviolet light. Also, because it is the glass with extremely low thermal expansion coefficient, it is the most suitable optical glass for use under high temperature.
While the light path is in air for a mirror, the light path remains in the glass for a prism. However, a permeable wavelength of any optical glasses is limited, and therefore only 2.3 or so is permeable on the long-wavelength side. Please use a mirror for angle-deflection applications on longer wavelength side (mid-infrared to far-infrared).

Various types of prisms are available, which have a wide variety of angular configurations. Among them, the regular triangle prism formed by the vertical angle of 60°has an effective shape especially for spectral diffraction applications of polychromatic light. Whatever vertical angle a prism has, a phenomenon of light dispersion occurs in a polychromatic light which enters the entrance plane obliquely due to difference in index of refraction of glass resulting from color (wavelength). (However, no light is dispersed when it is entered onto entrance plane in the form of vertical incidence). It is known that in this light dispersion phenomenon, a beam of light is dispersed at wider angle as the prism's vertical angle is wider or index of refraction of a glass used as a prism is larger. Well then, there may be some opinion that there should be a prism having wider vertical angle than 60 degrees to enhance this dispersion, whereas if the angle is greater than that, such disadvantages will result, to the contrary, that stronger effect will have on the prism's entrance plane or surface reflection on the outgoing light surface (Fresnel reflection), or that another strong effect will result from total reflection, which lead to poorer throughput. In view of the fact that the regular triangle prism is easy to be processed as well, it can be said that it is the most appropriate dispersing prism.

The optical window which takes the shape of parallel and flat substrate is used widely as incoming-light window or outgoing-light window for optical equipment or system. Because of that, various specifications are determined on the assumption that it will be placed in a boundary of the surrounding environment. Depending on a permeability characteristic of the substrate to be used, it is also used for the filtering to take out a beam of light at a certain wavelength only. Furthermore, it is used for protection window for Brewster's window and light emitter/detector, or as a protection filter of objective lens of imaging optical system. To select a proper optical window product, it is vital task to ascertain not only that a glass to serve as its substrate has translucency for a wavelength to be used, but also that consideration is given to specifications of the substrate's surface roughness, parallelism, etc.as well as to its glass's manufacturing process, characteristics, etc.

The optical window products made of glass can roughly be classified into what are fabricated by means of optical polishing process and what are fabricated by float method (float plate glasses) on a product-by-product basis. For materials used for optical polishing process, optical glass is generally used. For this reason, such optical glass is recommendable for high-precision optical applications because it is superior in homogeneity inside the glass and content of foam, foreign matter, etc. is at very low level. On the other hand, glass materials used for the float method which is the manufacturing process for architectural plate glass are not of optical glasses. On this account, such glass materials are used for condensing applications of polychromatic light source, standard imaging applications and so on which do not require a significantly high accuracy. However, the float method has another advantage in that it is also capable of manufacturing larger sized optical windows with ease. (Optical glasses are not suitable for manufacturing of larger sized products, because their block dimensions are limited.)

For the applications that are sensitive about wave surface (transmission wave surface) of a beam of light after passing through a window, including other applications for light interference experiments using a laser light source, etc., the window products having highly proper substrate surface roughness, which have undergone accurate optical polishing process, need to be selected. In this case, the surface roughness of 1/4λ or less is at least required (depending on application).On the contrary, however, for condensing applications of polychromatic light source and standard imaging applications, a float plate glass should suffice in nearly all cases. Those glasses that are fabricated by the float method generally have the substrate surface roughness of about 4 to 6λ (per 25.4 mm).

In an accurate measurement system using a laser beam, optical elements that have higher grade meeting the surface quality standards (generally 20 to 10 or below) are used. The reason is that scratch or pin hole, etc. on the substrate surface generate an unnecessary scattering, which mainly result in decrease in accuracy and S/N of the measurement system. On the contrary, however, for condensing applications of polychromatic light source and standard imaging applications, 60-40 levels or 80-50 levels should suffice in nearly all cases.

The parallelism that indicates a relative angle which makes the obverse side with the reverse side of substrate especially attaches importance to applications that are sensitive to a beam deflection angle, which are typified by the pointing applications using a laser beam. A degree of beam deflection angle can be approximated by (n - 1) x [parallelism] when the substrate's index of refraction is n at the wavlelength used.

Filters that are used for optical applications have the functions to selectively extract (pass) only part of information of a beam of light which is contained in an incoming light ray, and to absorb other pieces of information or cut off the same through reflection. Although there are various types of pieces of information in light information, they can be roughly classified into such three types as "wavelength", "polarized light" and "light intensity".

・A filter for wavelength is the filter that can selectively extract only a certain wavelength in an incoming light ray.

・A filter for polarized light is the filter that can selectively extract a certain component only in the direction of electric field oscillation from an incoming light ray. The polarization filter can be applied to such filter.

・A light-intensity filter is the filter that can change light intensity (amplitude) of an incoming light ray without depending on the aforementioned wavelength and polarization factors.　The ND filter serving as light extinction filter can be applied to such filter.

In addition to the filter products as stated above, we also offer wave plates (wave retarders) and light diffusion plates. In a strict sense, however, the wave plates and light diffusion plates cannot be classified as filters. That is because the wave plates and light diffusion plates change a state of a beam of light only, and thus the actual definition of a filter to the effect that the filter selectively extracts light information does not apply to them.

Most of the filters for wavelength can be broadly classified into the dielectric multilayer filter and the filter glass from the viewpoint of manufacturing. The dielectric multilayer filter is a type of filter where a dielectric multilayer to function as a filter is vapor-deposited on the substrate surface. The dielectric multilayer filter selectively extracts wavelength based on light interference effect. The dielectric multilayer filter is characterized by its property that it indicates a sharp rising edge (or falling edge) of pass/cut in a graph of spectral transmission characteristics.

However, because optical characteristics obtainable from the dielectric multilayer filter depend on an incident angle, it should be noted that it must be used carefully. In contrast, the filter glass extracts a certain wavelength through light absorption by means of the substrate itself. Because it does not depend on incident angle unlike the dielectric multilayer filter, it can be used with relative ease. On the other hand, however, it is characterized by its gradual rising edge (or falling edge) of pass/cut.

For a metal film used for mirrors, it is relatively easy to visually check to see if such film is put on which side of the substrate. By contrast, in the event that a dielectric multilayer having translucency as typified by an anti-reflection coating or a half mirror is vapor-deposited, it is sometimes difficult to identify on which side such coating is overlaid. The key to identifying it is to see the substrate's edge (round edge).The two pictures below show the IR cut filter and optical window, wherein the edge on the front side is indicated by a red arrow, and another edge on the back side is shown by a blue arrow. Through this difference in color or based on that the identical color is shown on the both sides, the coating surface can be identified. It can be seen from the pictures of IR cut filter that the surface is covered with coating because the edge on the side shown by a blue arrow appears to be tinted as compared with the edge on the side indicated by a red arrow. In the picture of optical window relative to them, because the on the side shown by a blue arrow, the same color appears as that of the edge on the side indicated by a red arrow, it can be seen that the film is not covered with coating. In the case of the products wherein the obverse and reverse sides of their substrates are covered with a coating having different and various optical characteristics, they should also be identified in a manner similar to the aforementioned point of view. By comparing color and brightness of the edge on the front and back sides with each other, each of their coatings can be identified.

Bandpass filter

The bandpass filter has the function to pass through only a certain wavelength band. It can broadly classified into a narrowband filter which passes through only the extremely narrow band (from 2 nm or less to 10 nm or so) and a broadband filter which passes through a wide band to a certain degree (from 50 nm to 80 nm or so).Because the optical characteristics of this filter is highly dependent on incident angle, extra special care should be taken when incorporating it into an optical system. As one of the available bandpass filters, there is the one which adopts a soft coating. Selecting this type of filter enables the transmissivity at the selected wavelength to be maximized. We do not offer this filter as the standard product, but it is available as custom-made article instead.

Color filter

This filter is provided with the function to absorb a certain band with respect to the visible spectrum (400 nm to 700 nm) which can be sensed by the human eye and to pass through other bands. Because it delivers the predetermined optical characteristics through light absorption of its substrate itself, an accurate filtering can be achieved by controlling the substrate's wall thickness. There are instances in which the color filter is used as a longpass filter or a shotpass filter. In addition, in comparison with a dichroic filter of coating type, the transition from the passband to pass-inhibition band is not as steep as that much. However, the peak transmissivity of color filter tends to be higher than that of dichroic filter. The optical characteristics obtainable from this filter do not depend on incident angle. We do not offer this filter as the standard product, but it is available as custom-made article instead.

Dichroic filter

	It has the function to reflect a certain band with a proper coating overlaid on the substrate, and to pass through other bands. Like the color filter, it is mainly designed for the visible spectrum (400 nm to 700 nm) which can be sensed by the human eye. Although the obtainable optical characteristics depend on incident angle, it does not depend on it as much as the bandpass filter does.

NDND filter

The neutral density filter (ND filter) is used for purpose of decreasing a certain light quantity without selecting wavelength at a specified wavelength. The ND filter can roughly be classified into two types; absorption type and reflection type. While the absorption type decreases transmitted light volume through light absorption of the substrate itself, the reflection type decreases the volume through light reflection effect. When using a reflection-type filter, care must be exercised not to make return light affect any application because of reflection. The ND filter is also often used for the applications to prevent saturation of cameras and other photoelectronic sensors.

Longpass filter

	It is the filter which has both the wavelength band blocking transmission and the passband which is a transmission wavelength range. The longpass filter passes all wavelengths that are longer than a certain cut-on wavelength. Cold mirror, filter glass, etc. are also classified as part of family of the longpass filter.

Shortpass filter

	It is the filter which has both the wavelength band blocking transmission and the passband which is a transmission wavelength range. The shortpass filter passes all wavelengths that are shorter than a certain cutoff wavelength. Hot mirror, IR cut filter, heat absorbing glass, etc. are also classified as part of family of the short pass filter.

The light is a type of electromagnetic wave, and its electric field is located in the plane which is perpendicular to the light travelling direction. Also, its direction of oscillation has the characteristics that it oscillates irregularly in all directions in the plane of natural light as typified by sunlight or general artificial light source. This oscillation state is called the "Unpolarized Light”. A polarization filter is used for the purpose of extracting only a certain oscillation direction (transmission axis) from an unpolarized state. As an application example of the polarization filter, the filter can also be used for an application similar to that of a concentration-variable filter by adjusting an angle made by each transmission axis of two polarization filters. In the meantime, however, it should be noted that a mark to indicate the location of transmission axis is not put on some of our polarization filter products. At the same time, however, it is easy to find out the transmission axis, as it can be identified easily by observing a glare from the surface of a shiny object through a filter. For example, a desk in the vicinity of window is a very good source for glare observation. Because, in the index of refraction of a glare reflected on desk surface which is classified by oscillation direction, the value of s component (electric field which oscillates in parallel with the desk surface) will be higher than that of p component (electric field which oscillates along the line normal to the desk surface). As a result, the former component appears to be the darkest due to the maximum light extinction by the action of filter upon the state in which the s component's oscillation direction becomes perpendicular to the filter's transmission axis, when it is observed while rotating the filter.

A wave plate (wave retarder) is the optical function element which provides a predetermined phase difference (retardation) to a beam of light of linear polarized light. As wave plates available in the marketplace, there are full-wave plate, 1/8 wave plate and so on, although two types of wave plates, half-wave retarder and quarter-wave retarder, are the ones for which needs are growing at the highest level.

Functions of half-wave retarders

The half-waver retarder (1/2λ plate) gives the phase difference of π (= λ/2) in the direction of electric field oscillation (to polarization plane) of an incoming light beam. When the incoming light beam's polarization plane is incident at the azimuthal angle of θ°with respect to the retarder's high-speed axis (or low-speed axis), its oscillation direction can be rotated (2 x θ°).Accordingly, when the incoming light beam is entered at the azimuthal angle of 45°, the maximum rotation angle (= 90°) can be obtained. This function is particularly advantageous for a laser light source, and when change in direction of the laser's polarization plane is desired, the plane can be moved only with a half-wave retarder. As another application of the half-wave retarder, the rotation direction of polarization of light can be reversed when a beam of circular polarization of light (circular polarized light) is entered.

Functions of quarter-wave retarders

The quarter-wave retarder (1/4λ plate) gives the phase difference of π/2 (= λ//4) in the direction of electric field oscillation (to polarization plane) of an incoming light beam. When the incoming light' beam's polarization plane is incident at the azimuthal angle of 45° with respect to the retarder's high-speed axis (or low-speed axis), linearly polarized light can be changed to circular polarized light. Reversibly, circular polarized light can also be changed to linearly polarized light .In the meantime, when the plane is incident at other azimuthal angle than 45°, a state of light changes to elliptically-polarized light. The quarter-wave retarder is often used with a polarization filter in a pair. This combined use enables an optical isolator to be configured. This wave retarder is used for the purpose of removing unnecessary return reflection and glare.

True zero order, multiple order, and compound zero order

While a wave plate (waver retarder) is fabricated using birefringent materials （crystals), such as crystal and mica, for its substrate, its design can roughly be classified into three types; True Zero Order, Multiple Order and Compound Zero Order. The wave retarder of true zero order is the "true" zero order wave retarder from which a specified retardation (phase difference) in the order of zero at design wavelength can be obtained. In order to obtain a certain phase difference in the order of zero, this wave retarder is fabricated in such a way that one board made of briefringent material is processed to have extremely-thin thickness. For example, when fabricating a quarter-wave retarder at 550 nm, the phase difference needs to be 137.5 nm (= 550 nm x 1/4).In a bid to obtain such phase difference from a crystal (double refraction: ne-no = 0.0092), the crystal has to be processed until its thickness becomes thin enough to the extent of 15 or so (137.5 nm/0.0092).However, this thin board thickness may involve greater degree of difficulty when securing the board to equipment or handling it. Except for this disadvantage, instead, a stability of phase difference which is obtainable relative to wavelength shift, temperature change or oblique light incidence is superior to that of other two design types.

The wave retarder of multiple order is so designed as to obtain a specified phase difference at a higher level for the purpose of thickening the board thickness to the extent of practical-use level, although it is also fabricated using a single board of briefringent material as is the case with the true zero order type. For example, when a phase difference which is equivalent to 3.25-long wavelength is generated at the wavelength of 550 nm, the board thickness can be increased to 194 or so if a crystal is used. This fact demonstrates that this phase difference of 3.25-long wavelength can be regarded virtually as the phase difference of 0.25-long wavelength (= 1/4).To the contrary, however, as the board thickness is increased, such a disadvantage will result that an nonnegligible phase lag to a small wavelength shift or temperature change, etc. occurs.

The wave retarder of compound zero order (sometimes simply called the "zero order" to distinguish itself from the true zero order) has the design that can improve the aforesaid disadvantage of the multiple order type. In a manner that optical axes of two boards made of the same birefringent material which are manufactured in the multiple order are disposed in a perpendicular relationship to each other, shift quantities of phase difference which are generated are offset each other. Consequently, wavelength dependency and temperature dependency on the retardation to be obtained can be lessened. Nevertheless, the incident angle dependency cannot be improved even by this design.

The ND filter is also known as "neutral density filter", which is used for the purpose of decreasing light quantity to a certain quantity without selecting any wavelength in a predetermined wavelength range.
For the ND filter, its properties are generally defined by optical density (OD), but not by transmissivity. The reason is that because they are defined by optical density, the entire attenuation rate (transmissivity) can be determined with greater ease when several filters are stacked on top of each other. There is a relation of the following expression between the optical density and transmissivity: 　
In the event that a different optical density from that of standard products is needed, the superposition of filters allows a specified density to be obtained. One instance thereof is cited as mentioned below.

(Example 1) When filters of OD = 0.3 and OD = 1.5 are stacked on top of each other;

(1) Just add up each OD value.
　　　ODtotal = 0.3 + 1.5 = 1.8

(2) The relational expression of optical density (OD) and transmissivity (T) is as follows:
　　　OD = log 10（1/T）=-log 10（T/100％）, or T=10^-OD

(3) Consequently, the transmissivity when the filters are stacked on top of each other will be as follows:
　　　T = 10^-1.8 ×100％=1.58%

(Example 2) When a filter whose transmissivity is 0.5% is required,
　　　the requirements are T = 0.005 to OD = log 10（1/T） = log 10（1/0.005） = 2.3.

Accordingly, the combination wherein the ODtotal will be 2.3 due to superposition of the filters (the combination of OD = 0.3 and 2.0, or OD = 1.0 and 1.3 for the case of the ND filter products mentioned below) allows transmissivity of 0.5% to be obtained.

The ND filter can broadly be classified into the refection and absorption types. The reflection-type filter is the one where a specified optical thin film (mainly metal film) is vapor-deposited on the glass substrate surface. By the reflex action and absorptive action of a light beam by means of this thin film, a specified optical density (transmissivity) can be materialized. However, because, in the case of reflection type, its amount of reflection is larger than that of the absorption type (as noted below), care should be taken, for example, not to return a return light beam to the outgoing-light outlet of laser light source when a laser beam is emitted. While for the case of absorption-type filter, it can provide a specified optical density (transmissivity) mainly by the light absorption action of the substrate itself. Please take notice that the ND filter products have not been designed for high power laser. (If they are used for high power laser, their filters may be damaged or their optical accuracy characteristics may be deteriorated.)