Technical Note:
Optical Filter Surface Quality

Surface quality and internal element quality are specifications which are used to define the level of defects to be allowed on the surface of an optical filter or within the element. Methods for specifying and inspecting surface quality have been defined by the U.S. Army in three commonly used military specifications; MIL-O-13830, developed in 1954 and revised to it's current level in 1963, MIL-C-48497, developed in 1974 and revised to it's current level in 1980, and MIL-F-48616, developed in 1977. These specifications categorize surface defects as either scratches or digs which can be defined as follows:

Scratch - A surface defect(tear) having length well in excess of width.

Dig - A surface defect(hole) having length approximately equal to width or being approximately circular in shape.

Scratches and digs are quantified by width and diameter, respectively, to allow for a universal specification language. In the newer military specifications, scratches are specified in units of 0.001mm, and digs, in units of 0.01mm. The commonly used scratch and dig sizes have been assigned letter designations. The tables that follow, set forth the commonly used specification values.

Scratches Digs
Scratch
Letter
Scratch
Width (mm)
Scratch
Number
Dig
Letter
Dig
Diameter (mm)
Dig
Number
A .005 5 A .05 5
B .010 10 B .10 10
C .020 20 C .20 20
D .040 40 D .30 30
E .060 60 E .40 40
F .080 80 F .50 50
G .120 120 G .70 70
H 1.00 100

A comprehensive surface quality specification will typically be stated as a scratch-dig combination and a military specification. The scratch-dig combination serves to define the maximum width and density of scratches and digs on a surface and the military specification the method of inspection and evaluation of the item under test. For example; F-F per MIL-F-48616 or 80-50 per MIL-O-13830.

In the first example, the specification allows a maximum scratch width of .080mm and a maximum dig diameter of .5mm and requires inspection as specified in MIL-F-48616. In the second example, the specified inspection method is detailed in MIL-O-13830. This specification requires comparison of observed defects to U.S. Army scratch-dig standards and therefore, no physical dimensions can be assigned to the maximum defect sizes. In both cases, the requirement is clearly stated. The method of inspection of surface quality is of critical importance. A comparison of MIL-O-13838 to MIL-F-48616 is provided below for reference

MIL-O-13830

Inspection Method

The element being inspected is viewed against a ground glass surface, illuminated from behind with a 40 watt bulb. The ground glass surface is fitted with opaque horizontal bars occupying 1/2 of the surface of the glass. These bars provide a contrasting surface facilitating the observance of light colored defects which would not be visible in the "light" area.

Inspection is conducted with the unaided eye. Observed defects are visually compared to U.S. Army scratch-dig standards to determine a subjective assessment of the defect size. Each surface is inspected and evaluated separately.

Evaluation method - Scratches

When a maximum size scratch is present, the sum of the products of the scratch numbers times their length divided by the diameter of the element, shall not exceed 1/2 of the maximum scratch number. When no maximum size scratch is observed, the sum of the products of the scratch numbers times their length divided by the diameter of the element, shall not exceed the maximum scratch number.

Evaluation method - Digs

No more than one maximum size dig per 20mm of diameter(or fraction thereof) is allowed. The sum of the diameters of all digs per 20mm of diameter shall not exceed twice the diameter of the maximum dig specified. Bubbles and inclusions shall be evaluated to the same specification limits as digs but shall be evaluated separately

MIL-F-48616

Inspection Method

The element being inspected is illuminated from behind or from above(depending on whether inspecting by transmission or reflection) with 2-15 watt fluorescent bulbs, viewed against a black matte background. Inspection is conducted with the unaided eye. Observed defects are measured to determine actual size. Each surface is inspected and evaluated separately.

Evaluation method - Scratches

When a maximum size scratch is present, the sum of the products of the scratch widths times their lengths divided by the diameter of the element, shall not exceed 1/2 of the maximum scratch width.

When no maximum size scratch is observed, the sum of the products of the scratch widths times their lengths divided by the diameter of the element, shall not exceed the maximum scratch width.

Evaluation method - Digs

No more than one maximum size dig per 20mm of diameter(or fraction thereof) is allowed.

The sum of the diameters of all digs per 20mm of diameter shall not exceed twice the diameter of the maximum dig specified. Bubbles and inclusions shall be evaluated to the same specification limits as digs but shall be evaluated separately

Each specification has its advantages. The inspection method of MIL-F-48616 provides better illumination of defects and the ability to measure defect sizes removes the subjectivity from the inspection. The dig comparison standards required by MIL-O-13830 provide a quick reference for the inspector to determine whether a defect is clearly acceptable without taking a measurement.

We routinely perform surface quality inspection to both of the above mentioned military specifications as required by customer product specifications. All of our custom products, where no specification has been provided by the customer, will be inspected per MIL-F-48616, modified to allow the use of comparison standards when appropriate. However, actual defect measurement will be considered as the prevailing factor in marginal pass/fail determinations. Under this inspection protocol, we will interpret scratch width numbers and letters consistent with MIL-F-48616 and MIL-C-48497, i.e., scratch width in microns