Understanding Optical Specifications: Surface Specifications

Introduction:

Optical specifications are critical parameters that define the functions and performance of an optic. Whether you are a buyer, a designer, or a manufacturer of optical components, understanding how to read optical specifications and knowing the concepts and definitions of these important glossaries and technical terms is pivotal. Optical specifications can be categorized into three main subcategories: Geometric Specifications, Surface Specifications, and Material Specifications/Properties.

In the previous article, we covered Geometric Optical Specifications (you can click the bold text to learn more if you are interested). This article offers a simple but thorough explanation of Surface Specifications, which is an essential subset of optical specifications and which include Surface Quality, Surface Flatness, Surface Roughness, Power, Irregularity, and Fracture/Chip. This knowledge will help you navigate the complexities of optics terms and enhance your grasp of optics concepts. We have also generated a new update introducing the Optical Material Properties.

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Optical Specification Glossary: Surface Specifications

• Surface Quality
• Surface Flatness
• Surface Roughness
• Power
• Irregularity
• Fracture/Chip

Surface Quality

Surface Quality refers to the imperfections on the surface of an optical component including pits, digs, scratches, and other defects that might affect the performance of the optic. In some circumstances, these imperfections bring negligible impact on the performance of the optics, but on some surfaces, these imperfections can have a detrimental influence and impede the normal functioning of the optical components. Typical examples include 1)surfaces at image planes, as these defects are in focus, and 2) surfaces exposed to high power sources. The most common standard used to measure and describe surface quality is the scratch-dig specification defined via MIL-PRF-13830B, which quantifies the maximum allowable scratch width and dig diameter. 

The scratch designation MIL-PRF-13830B is not a direct description of the scratch but is obtained by comparing scratches on the test surface to a set of standard scratches under controlled lighting conditions, hence it is a relative value. The dig designation is a direct description of dig and can be calculated at the diameter of the dig in microns divided by 10.

The Table for Precision Standards of Surface Quality:

Very Low	Low	Normal	Precision	High-Precision
80/50 S/D	60/40 S/D	40/20 S/D	20/10 S/D	10/5 S/D

Applications:

• Very Low: Acceptable quality. Largely available with low fabrication difficulties.
• Low: Commercial quality. Suitable for non-critical low-power laser and imaging, where cost weighs more than scattering loss. 
• Normal: Standard quality. Applicable for low to moderate power laser or imaging where little scattering is allowed.
• Precision: Precision quality. The lowest standard is required for laser mirrors and extra-cavity optical components with moderate to high power levels. Optimized to decrease scattering loss.
• High-Precision: High precision quality. Difficult to obtain. Selected for applications that necessitate the strictest scattering limits, such as intracavity laser optics or high-power scenarios.

Surface Flatness

Surface Flatness is the maximum deviation from a perfectly flat surface along the same lateral dimensions specified in a fraction of the reference wavelength. The deviation is measured by comparing optical flats of high precision and flatness as the reference surfaces against the test piece. Even, straight, and parallel fringes mean the flatness of the test piece is at least the same as the reference flat. In comparison, curved fringes indicate flatness error, and the number of curved fringes can be used to describe flatness error. Surface flatness is assessed using interferometers which provide a high-precision evaluation of the optic's flatness. The surface flatness specification is expressed in fractions of a wavelength of the test light source emitted from the interferometer (e.g., λ/4 or λ/10, and the standard wavelength of the test light source is usually 633nm), with one fringe equal to 1/2 wavelength. A surface with excellent flatness is essential for applications where wavefront distortion must be minimized. 

Surface flatness can also be known as surface figure, which is a measure of the deviation of the surface on the largest scale, as in comparison with waviness, the irregularities of the surface on the medium scale, and surface roughness (surface finish), the smallest scale surface error measured with the highest spacing frequency. Sometimes, the word “surface figure” includes both surface flatness and waviness. It is important to differentiate that errors on the low spatial scale tend to transfer light from the center of the airy disk pattern into the first few diffraction rings. As a consequence, the magnitude of the point-spread function (PSF) is reduced without widening it as well as the Strehl ratio. On the other hand, errors on the mid-spatial scale or small-angle scatter will widen or smear the PSF and reduce contrast. Both errors can degrade the optical system performance and are usually measured to ensure that specifications are met. Although some figure imperfections can be omitted from a surface-figure specification (i.e. astigmatism). 

The Table for Precision Standards of Surface Flatness:

Normal	Precision	High Precision
50 Å RMS	20 Å RMS	5 Å RMS

Surface Roughness

Surface Roughness, also called Surface Finish, is defined as the irregularities and deviations from its ideal surface smoothness. Surface roughness is the smallest form of surface shape error and describes high-spatial frequency abnormalities in the surface texture on the order of tenths of Angstroms to tens of µm. Talking of the spacing of errors, there are also two other concepts of “surface figure” and “waviness”. In comparison, the surface figure describes the overall shape of the surface and is the largest scale, or largest spatial frequency, which will be analyzed. The errors described by the figure are on the order of tenths of mm to cm. Waviness measures mid-spatial frequency errors describing features on the order of µms to mm. Roughness is the smallest form of error and describes closely-spaced abnormalities in the surface texture on the order of tenths of Angstroms to tens of µm.

Surface roughness is crucial for applications where scattering of light can degrade performance, such as in high-precision laser systems or imaging optics. High surface roughness also has an adverse effect on the laser damage threshold. The current criteria utilized which defines how surface roughness should be analyzed and specified is ISO 10110-8. The Root Mean Square (RMS) method is the most common method in which optically smooth surfaces are specified in the United States. The RMS average of profile height deviations from the mean line is utilized to complete a statistical analysis of the smoothness of an optical surface. Atomic force microscope is the most suitable metrological equipment to measure surface roughness.

The Table for Precision Standards of Surface Roughness/Surface Finish (presented in RMS):

Normal	Precision	High Precision
50 Å RMS	20 Å RMS	5 Å RMS

Power

Power in optical specifications describes the deviation of curvature of the optical surface from its intended curvature. Power error is applicable to optics with curved profile, or optics with power and is important in the aspect that ensures the optical component converges or diverges light according to the engineered functions. The test principle is similar to that of surface flatness, in which an optical surface with a high-precision radius of curvature is used as the reference piece, and placed against the test piece. Deviation from the reference piece creates circular interference fringes, known as Newton’s Rings. The number of rings indicates the difference in radius between the two surfaces, which is known as power. The separation between each fringe of the same color (dark or bright) represents a height difference of half a wavelength of the light used, therefore the number of dark or light rings is equal to twice the number of waves of error.

The relationship between the error in radius of curvature and power error can be described using the formula below:

Power Error (in waves or λ)=ΔR*D^2/8R^2*λ

Where ΔR represents the radius error, D represents the lens diameter, R represents the surface radius, and λ represents the wavelength of the test light source (usually 633nm)

Irregularity

Irregularity refers to the deviation of an optical surface from the desired shape, excluding the overall curvature (power). Surface irregularity is measured using the method of measuring power, where placing the test plate and the surface under test into close contact creates circular interference fringes, known as Newton’s Rings, the rings might exhibit distortions that might be local in a small area or might be in the form of non-circular fringes across the entire aperture. All such non-uniformities are considered as irregularity. Surface irregularity is often specified in terms of Peak-to-Valley (PV) or Root Mean Square (RMS) values. It is worth noting that while measuring the irregularity one must state whether the measurement is done on the surface or the wavefront since the measurement on the wavefront is twice the measurement on the surface. The PV value is the maximum measurement and the worst-case scenario, considering the difference between the surface’s lowest and highest points. It is by far the most widespread flatness specification used today. A more accurate measurement of surface flatness is RMS as it considers the entire optic and calculates deviation from the ideal form. High irregularity can introduce aberrations and distort the wavefront, affecting the optical performance.

The Table for Precision Standards of Irregularity:

Low	Normal	High Precision
λ/2	λ/4	λ/8

Applications:

• Low:  Chosen when wavefront distortion is not as important a factor as production cost.
• Normal: Sufficient for common laser and imaging applications where the existing wavefront distortion is still acceptable considering the cost.
• High precision: Minimized wavefront distortion, for high-demanding lasers and imaging applications, particularly in systems with multiple elements.

Fracture and Chips

Fractures that do not appear inside the range of clear aperture of the lens are acceptable, provided that the chips do not obstruct the sealing of the lens in its mount. Fractures on all faces or edges need to be ground out. The total area of ground chips and fractures in the ground areas must not exceed 2% of the ground area; otherwise, the optics are disqualified. Edge chips that intrude into the clear aperture area shall be considered digs. Chips larger than 0.5 mm must be ground to reduce the potential risk of reflections and further chipping. The total width of chips larger than 0.5 mm must be maintained within 30% of the lens perimeter. Edge chips that do not encroach on the clear aperture of the prism are allowable as long as the total width of the chips is within 30% of the edge length where the chips exist. Chips shall be measured from the beveled edge rather than the sharp edge. Chips smaller than 0.5 mm can be omitted, while those greater than 0.5 mm need to be stoned or ground.

How to Read Optical Specifications

Reading and interpreting optical specifications involves understanding the relevant optics terms and their explanations. Here’s a quick guide:

1. Refer to the Optical Specification Glossary: Use an optical specification glossary to understand the definitions of terms.
   
2. Evaluate the Parameters: Assess the given parameters against the required performance criteria for your application.
3. Consult Standards: Check industry standards such as ISO or MIL-PRF for detailed definitions and acceptable tolerances.
4. By mastering these optics concepts, you can ensure that the optical components you design, manufacture, or purchase meet the necessary performance requirements.

Hangzhou Shalom EO’s Metrology

Understanding optical specifications is fundamental for anyone involved in the optics field. This article has provided a detailed explanation of surface specifications, including surface quality, flatness, roughness, power, irregularity, and fractures/chips. These parameters are crucial for the performance and functionality of optical components. 

      

Tests using Zygo interferometer and microscope in Shalom EO

Hangzhou Shalom EO has a matured ISO 9001-certified quality control system. In our Class 100000 Cleanroom inspection lab, all the products will undergo stringent supervision before dispatch to ensure the actual parameters of products meet or exceed the published specifications. 

For the visual error, the scratches/digs are identified by qualified personnel with developed skills and trained experiences, under the observation of an intense light source with a 10x microscope.

The dimensions of the product are measured using vernier calipers and micrometer screws to ensure the actual dimension remains within the required tolerances. The vernier calipers and micrometer screws are calibrated on a routine basis so that the equipment stays correct.

For the surface flatness errors, power, and parallelism, reliable equipment--Zygo interferometer is utilized. 

An atomic force microscope is utilized to obtain authentic measurements of the surface roughness.

Tags: Understanding Optical Specifications: Surface Specifications

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