Military Specifications
Optical Infrared Interference Filters
IR Filters: 1.5 – 20 μm
The optical coating industry has adopted the US Military Specifications (often referred to as MIL specs) as a standard for testing the mechanical properties of coatings. All NOC coatings are inspected for surface quality (standard scratch – dig, 80-50 or FF) and tested for adhesion. NOC can also test for humidity, abrasion or other MIL specs to customer requirements.
Surface Quality of Coated Components
Scratch and Dig Check
MIL-F-48616 – Scratch and Dig (section 4.6.7.2)
The coated substrate shall be examined for scratches and digs by reflection or transmission, as applicable, using the inspection technique specified in 4.6.4. Magnification shall be used as needed in these examinations. The length and width of scratches, and the dig (hole) diameters shall be determined by use of interferometer, microscopic measuring devices, calibrated precision comparators, or similar applicable precision measuring devices. The width, length and density of all scratches in the coating or substrate shall conform to the requirements of 3.4.1.2 and 3.4.1.2.1. The diameter and density of all digs (holes) in the coating or substrate shall conform to the requirements of 3.4.1.3 and 3.4.1.3.1.
3.4.1.2 Scratches. – Surface scratches (coating and substrate) shall not be in excess of the values specified on the component drawing or procurement document (See 6.2f). Scratches are permissible provided the width does not exceed that specified by the scratch letter. The accumulated length of all maximum scratches shall not exceed 1/4 of the average diameter of the element, The scratch letter and corresponding width are shown in Table I.
TABLE I: Scratch Identification (See 6.3.1)
| Scratch Identification | |||||
|---|---|---|---|---|---|
| Scratch Number (per MIL-PRF-13830B) | Scratch Letter | Scratch Width | Disregard Scratch Widths less than | ||
| Millimeters | Inches | Millimeters | Inches | ||
| 5 | A | .005 | .00020 | .0010 | .00004 |
| 10 | B | .010 | .00039 | .0025 | .00010 |
| 20 | C | .020 | .00079 | .0050 | .00020 |
| 40 | D | .040 | .00157 | .0100 | .00039 |
| 60 | E | .060 | .00236 | .0100 | .00039 |
| 80 | F | .080 | .00315 | .0200 | .00079 |
| 120 | G | .120 | .00472 | .0200 | .00079 |
3.4.1.3 Digs. – Surface digs (coating and substrate) shall not be in excess of the values specified on the component drawing or procurement document (See 6.2g). Digs are permissible on a surface provided the average diameter does not exceed that specified by the dig letter and no more than (1) maximum size dig occurs in any 20mm (0.8″) diameter circle on the substrate. The dig letter and corresponding average diameter are shown in Table II.
TABLE II: Dig Identification (See 6.3.1)
| Dig Identification | |||||
|---|---|---|---|---|---|
| Dig Number (per MIL-PRF-13830B) | Dig Letter | Average Dig Diameter | Disregard Digs Smaller Than | ||
| Millimeters | Inches | Millimeters | Inches | ||
| 5 | A | .05 | .0020 | .010 | .0004 |
| 10 | B | .10 | .0039 | .025 | .0010 |
| 20 | C | .20 | .0079 | .050 | .0019 |
| 30 | D | .30 | .0118 | .050 | .0019 |
| 40 | E | .40 | .0157 | .100 | .0039 |
| 50 | F | .50 | .0197 | .100 | .0039 |
| 70 | G | .70 | .0276 | .200 | .0079 |
| 100 | H | 1.00 | .0394 | .250 | .0099 |
6.3.1 Scratch and dig identification. – This is specified by two letters separated by a hyphen (i.e. F-F). The first letter of the pair is the maximum scratch letter. The second letter is the maximum dig letter.
Environmental and Physical Durability
Adhesion Test
MIL-F-48616 – Adhesion (section 4.6.8.1)
The coated component, or witness piece, shall be subjected to an adhesion test using 1/2″ (12.7 mm) wide cellophane tape conforming to type I of L-T-90. Press the adhesive surface of the cellophane tape firmly against the coated surface so as to cover the stained area, then quickly remove it at an angle which is normal to the coated surface. Immediately following the removal of the adhesive tape, the coated surface of the component, or witness piece, shall be examined by reflection using the inspection technique specified in 4.6.4. The coating shall conform to the requirements of 3.4.2.1.1. Subsequent to this test the coated component, or witness piece, shall be subjected to the test in 4.6.8.2.
Humidity Test
MIL-F-48616 – Humidity (section 4.6.8.2)
The coated component, or witness piece, shall be placed into an environmental controlled test chamber and exposed to a temperature of +120° +/-4°F (49° +/-2°C) and 95% to 100% relative humidity for a minimum of 24 hours. Subsequent to this exposure the coated component, or witness piece, shall be removed from the test chamber, cleaned (see 4.6.1), dried, and then subjected to the examinations specified in 4.6.7.1 and 4.6.7.3. The coating shall conform to the requirements of 3.4.2.1.2. The coated component, or witness piece, shall then be subjected to the test specified in 4.6.8.3.
Moderate Abrasion Test
MIL-F-48616 – Moderate Abrasion (section 4.6.8.3)
Within one hour after the humidity test of 4.6.8.2 the coated component, or witness piece, shall be subjected to a moderate abrasion be rubbing the coated surface with a 1/4 inch (6.4mm) thick by 3/4 inch (9.5mm) wide pad of clean dry, laundered cheesecloth conforming to CCC-C-440 affixed to an abrasion tester that conforms to drawing D7680606.
The cheesecloth pad shall completely cover the eraser portion of the tester and be secured to the shaft with an elastic band. The cheesecloth pad shall be rubbed across the coated surface from one point to another over the same path for 25 complete cycles (50 strokes) with a minimum force of 1.0 pound (0.45kg) continuously applied. The length of the stroke shall be approximately equal to two widths of the cheesecloth pad when the diameter of the area of the component permits.
The abrasion tester shall be held approximately normal to the surface under test during the rubbing operation. Subsequent to the rubbing operation the component, or witness piece, shall be cleaned (see 4.6.1), dried and then subjected to an examination by reflection using the inspection technique specified in 4.6.4 for evidence of physical damage to the coating. The coating on the component, or witness piece, shall meet the requirements of 3.4.2.1.3.
Northumbria Optical Coatings Ltd (NOC) was established in 1995, specializing in the design and manufacture of Infrared Optical Filters and Coatings, covering the spectral range 1.5μm – 20.0μm. Before 1995 NOC operated as a business unit for Rolls Royce International Research & Development based in Newcastle upon Tyne. NOC offers competitively priced standard and non-standard filters which are used in many applications including environmental monitoring, analysis and control of gaseous effluent, analysis of vehicle exhaust gases, ozone layer monitoring, night vision systems and space applications. Contact EOC for more information.