Optics/Interferometers & Others

Optics/Interferometers & Others

Technical note

Many of our optics (optical elements) are classified into two categories depending on their grades; high-quality and standard-quality categories. This grading enables our individual customers to identify our products quickly in accordance with their intended use. The differences in their categories based on their grading as well as their features are as follows:
High-quality categories
・ Recommendable for OEM/Mass-production (available for continual supply)
・ High quality and high performance

Because they are available for OEM/mass production as well as for research application, information on very wide variety of optics is provided in this catalog, such as lens, prism, mirror and optical window.

■ Continual supply:
A rapid production and supply together with a long-term and continual supply can be effected. For this reason, these optics are best suited not only for prototype fabrication and development, but also for approach to OEM/mass production.

■ Systematic range of products:
Because our product range is based on systematized standards, a systematic work operations can be performed for research projects and design development.
Standard-quality categories
・Limited number of pieces of technical data
・General-purpose design

The optics in the standard-quality categories are the first-class products that meet the standards required for general-purpose products, and thus they can always be supplied quickly even to deal with inquires for large-volume production. As their accuracy and quality are standardized, they can usually comply with a request for lump-sum purchases in large quantities for mass production. Unlike the optics in the high-quality categories, their product range is not systematized.
They are also characterized by a narrow range of specification settings.

RoHS Directive and optics

What is RoHS Directive (2002/95/EC)?
From the date of July 1, 2006 onward, specified electric and electronic equipment (mainly consumer equipment for individual consumers) sold in the markets of EU member countries are subject to RoHS [Restriction (of the use) of Hazardous Substances] application. In accordance with this directive, following six toxic chemicals are restricted from usage. Specified six toxic chemicals include lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chrome (Cr [VI]), and polybrominated diphenyl ether (PBDE) and polybrominated biphenyls (PBB), which are flame retardant plastic. This directive is one of the EU Environmental Directive series and is closely related to WEEE (Waste Electrical and Electronic Equipment) directive.
What happens if any one of these specified six materials is used in the product?
Under RoHS Directive, the products which includes material exceeding the allowed maximum concentration (ratio to the weight) must not be sold (effective from July 1, 2006 onward).
The allowed maximum density is 100 ppm (0.01%) for Cadmium and 1000 ppm (0.1%) for the rest of materials. (However, the allowed density is higher in some subject materials.)
Do our optics comply with RoHS Directive?
It is believed that our glass optics (optical elements) described in this catalog can meet the requirements under RoHS Directive in accordance with conditions for the allowed maximum concentration which have been announced officially by EU's TAC (Technical Adaptation Committee) or Annex 13 of 2005/747/EC Directive. As the latter 2005/747/EC's Annex 13 approves exemption from application of such requirements to glass optics and filter glass products for optical application, and allows the use of lead and cadmium which are often contained in such products. As a result, these products "comply with" the RoHS Directive (2002/95/EC Directive).

Description of symbols shown in lens figures

f (Effective Focal Length): (有効)焦点距離
F.f (Front Focal Length): フロントフォーカス
B.f (Back Focal Length): バックフォーカス
P (Primary Principal Point): 前側主点(第一主点)
P (Secondary Principal Point): 後側主点(第二主点)
t1 (Edge Thickness): コバ厚
t2 (Center Thickness): 中心厚
R1 (Radius of Curvature): 第一面の曲率半径
R2 (Radius of Curvature): 第二面の曲率半径

Various characteristics of optical glass

( ※) @589.3 nm (☆)Birefringent materials. The specified values are effective when the material is parallel to axis C. (★) Maximum operating temperature
Name of equipment Index of Refraction; nd Abbe Number; Vd Specific gravity; ρ (g/cm3) Thermal expansion coefficient; α (x 10-6/°C) Transformation Temperature; T (°C)
Calcium fluoride (CaF2) 1.434 95.10 3.18 18.85 800★
Synthetic quart 1.458 67.70 2.20 0.55 1000★
BorofloatTM 1.472 56.70 2.20 3.25 450★
Pyrex R7740 1.474(※) 65.4(※) 2.23 3.20 490★
N-BK7 1.517 64.20 2.46 7.10 557
N-K5 1.522 59.50 2.59 8.20 546
B270/S1 1.523 58.50 2.55 8.20 533
ZerodurR 1.542 56.20 2.53 0.05 600★
N-SK11 1.564 60.80 3.08 6.50 604
N-BaK4 1.569 56.10 3.1 7.00 555
N-BaK1 1.573 57.55 3.19 7.60 592
L-BAL35 1.589 61.15 2.82 6.60 489
N-SK14 1.603 60.60 3.44 7.30 649
N-SSK8 1.618 49.80 3.33 7.10 598
N-F2 1.620 36.40 3.61 8.20 438
BaSF1 1.626 38.96 3.66 8.50 493
N-SF2 1.648 33.90 3.86 8.40 441
N-LaK22 1.651 55.89 3.73 6.60 689
S-BaH11 1.667 48.30 3.76 6.80 575
N-BaF10 1.670 47.20 3.76 6.80 580
N-SF5 1.673 32.30 4.07 8.20 425
N-SF8 1.689 31.20 4.22 8.20 422
N-LaK14 1.697 55.41 3.63 5.50 661
N-SF15 1.699 30.20 2.92 8.04 580
N-BaSF64 1.704 39.38 3.20 9.28 582
N-LaK8 1.713 53.83 3.75 5.60 643
N-SF18 1.722 29.30 4.49 8.10 422
N-SF10 1.728 28.40 4.28 7.50 454
S-TIM13 1.741 27.80 3.10 8.30 573
N-SF14 1.762 26.50 4.54 6.60 478
Sapphire☆ 1.768 72.20 3.97 5.30 2000★
N-SF11 1.785 25.80 5.41 6.20 503
N-SF56 1.785 26.10 3.28 8.70 592
N-LaSF44 1.803 46.40 4.46 6.20 666
N-SF6 1.805 25.39 3.37 9.00 605
N-SF57 1.847 23.80 5.51 8.30 414
N-LaSF9 1.850 32.20 4.44 7.40 698
N-SF66 1.923 20.88 4.00 5.90 710
S-LAH79 2.003 28.30 5.23 6.00 699
Zinc selenide(ZnSe) 2.403 N/A 5.27 7.10 250★
Silicon(Si) 3.422 N/A 2.33 2.55 1500★
Germanium(Ge) 4.003 N/A 5.33 6.10 100★
(*) For the Index of Refraction (nd) and Abbe Number (Vd), the d line (587.6 nm) which is the yellow spectral line emitted from a helium light source serves as an optical reference wavelength. They indicate various characteristics based on the reference wavelength. The definitions of each of them are as follows:
For your information, nC and nF, which are coefficients of Abbe Number, are the index of refraction of the c line (656.3 nm) and F line (486.1 nm) which are the spectral lines of hydrogen.

(**) The Transformation Temperature (Tg) refers to a temperature at which a glass transforms into a viscoelastic state from a rigid state, which is used as a reference guide for maximum operating temperature as mentioned below.

[Tg-300℃]: Temperature not to be exceeded during a long-time use
[Tg-250℃]: Temperature not to be exceeded during a short-time use
[Tg-200℃]: Temperature not to be exceeded by all means during use

■ Description of optical glass (code numbers)
This catalog describes the information that indicates a glass's index of refraction and Abbe number in accordance with the MIL-G-174 standard. As an example, 517/642 is described here for the N-BK7 glass, because its index of refraction (nd) and Abbe number (Vd) are 1.517 and 64.2, respectively.
A lot of optical glasses have been used for the optics described in this catalog. Because each of the glass materials has different optical characteristics, there are instances in which selection of the materials becomes important. In particular, the index of refraction (nd) and Abbe number (Vd) are the data which are used on a frequent basis. Also note that many optical glass manufacturers manufacture and sell their glass products under their own names which have almost the same optical characteristics as those of our products. In order to supply our glass products to our customers rapidly for production of optical components, we regard these glass products as the same as those of our products and procure materials from several glass manufacturers for us. It is almost improbable that any difference in glass products among glass manufactures could affect our optical components.

Differences in optical performances among lens systems

All the optical systems (lens systems) using lenses can be roughly classified into the following three types of design. They are the Finite Conjugate Design (finite-system design), Afocal Design and Infinite Conjugate Design (infinite-system design).
What are introduced to this catalog are various types of lenses which are available for different shapes of lenses. They include the single lenses as exemplified by a plano convex lens and a double convex lens, and the laminate lenses (achromatizing lenses) as typified by an achromatic lens. All of these lenses can constitute the aforementioned three types of design by using a single lens or several lenses. The charts shown below indicate a rough measure for merits and demerits or the like which result from a constitution of these three types of design through the use of each lens.
▼How to understand charts
As factors to evaluate a design which is configured with each lens, such three items as low F number, pleochromatism and field-of-view efficiency are specified as the evaluation criteria.
The evaluation was made ranked on a 5-point scale based on ★ mark (★= lowest, and ★★★★★= highest).

■ Low F number:
It is the light collection capability when an F number is low (when a diameter to capture light is large, or when numerical aperture [NA] is high).The lenses that can be used for low-F-number applications can be evaluated on the basis of (★★★★★).

■ Pleochromatism:
It is the light collecting capability relative to polychromatic-light illumination (white-light illumination).Unlike a monochromatic light source including laser, LED, etc., evaluation criterion for the polychromatic-light illumination is presence or absence of chromatic aberration. Those lenses that function effectively without using a filter in polychromatic-light illumination are evaluated on the basis of (★★★★★).

■ Field-of-view efficiency:
It is the capability to deal with a large-sized light source (or object).Alternatively, it is the capability to deal with a large-sized sensor (or image).In the Afocal Design, it refers to the ability to support a wide angle of field. Those lenses that deal with larger size are evaluated on the basis of (★★★★★).
1. Finite Conjugate Design
It refers to a design in which a beam of light from an object located at a finite position but not at an infinite distance can be focused onto another one point via optical system. For imaging applications, most of the camera lens products fall under the category of this finite conjugate design. It aims to produce an image of visual information of an object (a photographic subject) located at a finite position on a sensor of the camera which is located at the same finite position.

Common application: Camera imaging optical system, relay lens optical system and image projector

■ Single-lens configuration:
It is the simplest configuration consisting of only one lens. In the event that a finite conjugate design's optical system is configured with only one lens, a focal length of the lens itself which is used will be a focal length of optical system. The advantages of this configuration are less investment costs and a simple configuration. How to select a lens with what focal length can be found from the formula mentioned below.

■ Two-lens configuration:
Combination of two lenses with different focal lengths (the same focal length is acceptable) enables the light collection performance to be improved significantly more than the single-lens configuration. In this configuration, a focus of the lens located on the side of object will be an object point, and a focus of the lens located on the side of image will be an image point. As compared with the single-lens configuration, its configuration is a little less simple.

■ Realistic proposal:
For imaging applications, lamination lenses, such as achromatic lens, are usually used. The reason is that use of lamination lenses will result in a higher image quality. Single lenses, such as plano convex lens and double convex lens, are generally used for non-imaging applications (e.g., illumination optical system, etc.).
That is because no better resolution is normally required for non-imaging applications.
2. Afocal Design
It refers to a design in which a beam of light (called "parallel light" or "collimated light") from an infinite distance is emitted as a parallel light of a different size by means of the optical system having a specified magnification. Telescopes for imaging applications and beam expanders for condensing applications fall under the category of this design.

Common application: Telescopes and beam expanders

■ Two-convex-lens configuration:
Two-convex-lens configuration has such an advantage that an image is produced once by an previous-stage lens. Disposing a reticle, such as cross-hair scale, at this image point permits a subsequent-stage lens to focus the beam onto both the reticle and image. On the contrary, its disadvantage is that this configuration brings about a negative magnification (an inverted image).On this account, if an erecting image is required, some contrivance is required to use a prism concurrently or locate a relay lens between two lenses in the two-lens configuration.

■ Configuration consisting of one convex lens and one concave lens:
A laser beam expander is usually designed in this configuration.
In comparison with the two-convex-lens configuration, this configuration has advantages in that a system length of the entire optical system can be shortened and an erecting image can be obtained.

■ Realistic proposal:
Because the filed-of-view efficiency can be improved, lamination lenses, such as achromatic lens, are usually used.
In the meantime, for ocular lens, the afocal design can be adopted sometimes as a means to improve the image-formation performance.
3. Infinite Conjugate Design
This design is a mixture of two designs of the finite conjugate design and afocal design, which refers to a design in which a beam of light from an object located at an infinite distance is focused onto one point. Also, in a reversible fashion, such an optical system that a beam of light from an object located at a finite position is converted to an infinitely distant light (parallel light) also falls under the category of this design. Auto collimators and infinity-corrected objective lenses designed for microscopes for imaging applications and the laser focusing for condensing applications fall under the category of this design.

Common application: Auto collimator, optical detector and infinity-corrected objective lens

■ Single-lens configuration:
Except for the infinity-corrected objective lens, multiple-lens-configuration is not required for general cases. This design is mainly used for single-lens configuration. In this use, a focus of the lens which is used will be an image position. As important factors of the finite conjugate design, there are the F number which qualitatively shows a light collecting capability of lens and the numerical aperture (NA) which qualitatively shows a resolution of collected light. Individual F numbers and numerical apertures based on optical system can be calculated from the formula
mentioned below.

■ Realistic proposal:
With a single lens, better results can be obtained for the focusing applications of most of the parallel light beams.
Description of symbol
Hi, Ho Hi means an image height, and Ho denotes an object height. Both of these heights are defined by a half value (height from optical axis) of actual size. Additionally, in the afocal design, they are defined by a half value of beam waist of laser.
I, O I means an image distance, and O denotes an object distance. Both of these distances are defined by a distance from the lens's principal point position (posterior principal point and anterior principal point for each distance).
Fi, Fo Fi, Fo: Focal length. For the optical system in two-lens configuration, Fi and Fo refer to each focal length of lens on the image side and object side, respectively.
f Effective focal length. For the optical system in two-lens configuration, this refers to a composite focal length of the entire system.
M Magnification of optical system. This refers to zoom in/out of an image or projection size of an object.
θ It is a cone angle of light which is accepted or emitted by optical system. It is indicated by total angle.
d For the optical system in two-lens configuration, this refers to a distance between two lenses.
f/# F number. It represents light collecting capability (brightness) of optical system.
D It means a diameter of lens.
αi, αo: It is an angle of field for optical system in the infinite conjugate design. It is indicated by half angle.

Anti-Reflection Coating; AR coating

The anti-reflection coating has the function to reduce unnecessary return reflection and improve transmissivity. While there are obverse side and reverse side on a lens, the amount of surface reflection (Fresnel reflection) which is generated on one side depends on the index of refraction of a substrate through which a beam of light propagates and the index of refraction of media surrounding the substrate, which can be obtained from the theoretical formula shown in the right equation (for vertical incidence).
As an example of the theoretical formula shown in the right equation, for the case of a standard optical glass whose index of refraction (ns) is 1.5, the equation R = (0.5/2.5)2 = (0.2)2 = 0.04 holds, provided that no is equivalent to air's index of refraction (= 1); and it is clear that the reflection loss is 4% per plane (8% for both planes).When this is seen from the ZnSe lens of which index of refraction is 2.4, R equals as much as 17% and it is obvious that the reflection loss on the both planes is 34%.
Even for the lenses whose reflection loss is relatively low which is in the range of a few percentages or so, reduction in image contrast and generation of ghost image could be caused to the optical system using several lenses; and therefore such loss and ghost cannot be ignored from practical point of view. Adoption of optical components equipped with anti-reflection coating can significantly contribute to improvement in this kind of trouble.
▼ Anti-reflection coating for visible range
The MgF2 coating is the most typical coating among our anti-reflection coatings, where the anti-reflection coating is covered with a single layer film of MgF2 (magnesium fluoride) which is overlaid on a glass substrate. It is designed with the center wavelength of 550 nm (center wavelength of visible wavelength range), which limits the average index of refraction of the entire visible range from 400 nm to 700 nm to 1.75% or less (per plane).

A visible-range multi coating is a dielectric multilayer (multi coating) for visible range with lower index of refraction than the aforementioned MgF2 single coating. In the range from 425 nm to 675 nm, this coating reduces the average index of refraction to 0.4% or less (per plane).

The multi coating for visible range and near infrared ray region is a dielectric multilayer (multi coating) which achieves transmissivity at 98% level with respect to a wide band ranging from visible range to near infrared ray band.
▼ Anti-reflection coating for ultraviolet ray
The multi coating for ultraviolet ray region is a dielectric multilayer which achieves absolute reflectance of 1% or less (per plane) in the range from 250 nm to 425 nm.

The multi coating for visible range and ultraviolet ray region is a dielectric multilayer (multi coating) which limits the average index of refraction to 1.5% or less (per plane) in the range from 250 nm to 700 nm.
In particular, this coating reduces the index of refraction to 1% or less per plane of lens in the 350-450 nm range.(per plane)
▼ Anti-reflection coating for near-infrared ray
The multi coating for infrared and near infrared ray regions is a dielectric multilayer (multi coating) which achieves transmissivity at 99% level with respect to a wide band ranging from infrared ray band to near infrared ray band in the visible range. In the range from 600 nm to 1050 nm, this coating reduces the average index of refraction to 0.5% or less (per plane).

The multi coating for near infrared ray region is a dielectric multilayer which achieves transmissivity at 98% level with respect to the near infrared ray band. This coating limits absolute reflectance to 1% or less in the range from 800 nm to 1550 nm and the same to 1.5% or less in the range from 750 nm to 800 nm (per plane).Enhanced Reflectance Coating.

Enhanced Reflectance Coating

For the enhanced reflectance coating, a metal coating and a dielectric multilayer are available. Five kinds of metal coatings are available as standard reflectance coatings from our product lineup.
In each coating, the metal coating surface is covered with a dielectric protection film, which results in prevention against deterioration of reflectance properties due to oxidation of metal coating itself and enhancement of durability of the coating itself.
It also promotes the ease with which cleaning can be performed.
▼ Metal coating for enhanced reflectance
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 400 nm to 700 nm.

The enhanced aluminum coating (Enhanced Aluminum) makes improvements in the average index of refraction of visible light in the range from 450 nm to 650 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 because its overall index of refraction is extremely high in the visible range.

The UV enhanced aluminum coating (UV Enhanced Aluminum) 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 250 nm to 700 nm, this coating can achieve the average index of refraction of 85% or higher.

The gold coating (Protected Gold) 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 700 nm to 800 nm and from 800 nm to 2 μm, respectively. As is the case with the aluminum coating (noted above), the silicon monoxide (SiO) is used as a protection film.
Note, however, that handle this coating with care when cleaning and wiping it clean manually because mechanical strength of gold is very low.

The silver coating (Protected Silver) provides the higher index of refraction (98%) than any other metal coatings in the range from 500 nm to 800 nm. However, this coating is prone to tarnish, and it is best suited for it to be used as the enhanced reflectance coating on the reverse side of mirror. 
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