A US Department of Defense performance specification dated 9 January 1997, covering the manufacture, assembly and inspection of finished optical components such as lenses, prisms, mirrors, reticles, windows and wedges, for fire control instruments. It supersedes MIL-O-13830A of 11 September 1963.
It is best known for one paragraph — the scratch and dig numbers of section 3.5 — but that is a small part of an 81-page document that also governs glass, chamfers, cementing, coatings, resolution, concentricity, edge chips and how every one of those is to be inspected. This page walks the whole specification, naming the governing clause throughout, and carries the document itself as a PDF.
Six numbered sections, followed by five appendices that are each a mandatory part of the specification in their own right.
Names what the specification covers: finished optical components for fire control instruments.
The documents that form part of the specification — including Drawing C7641866, the surface quality standards.
The substance: materials, dimensions, finish, surface quality, cementing, component details and optical systems.
Who inspects, and by what method — a test procedure for nearly every requirement in section 3.
Packaging is left to the contract or order rather than fixed by the specification.
Explicitly non-mandatory: intended use, what an acquisition document should state, and the definitions.
All optical elements, components and systems shall comply with the requirements of this specification, except as further defined in the detailed instrument specification or on applicable drawings forming a part of the contract. 3.1
Optical glass shall be of the type and grade specified on the drawings, and shall contain no thorium or other added radioactive material in excess of 0.05 percent by weight. Cements, bonding systems, sealants and coatings are each handed to an appendix.
Rim edges of all optical parts shall have a chamfer of 0.020 inch - 0.010 at 45 degrees ± 15 degrees measured along the face width. Edges meeting at angles of 135 degrees and larger need not be beveled unless the drawings say so.
Striae, cords, ream, bubbles, seeds, strain, laps, folds in pressings, or any other defect located in such a point, plane, or position as to impair the performance of the element, shall be cause for rejection of that element. 3.4.1
The scratch and dig requirements, and Table I. The most cited section of the document, and the one set out in full below.
Cement bubbles, voids, undissolved particles, dry spots, blisters and dirt within the free aperture are held to the dig and bubble limits of 3.5.3.1 through 3.5.4.1. The cement interface counts as a single surface of the specified surface quality; where none is specified it is intermediate between the adjacent faces. 3.6, 3.6.1 No edge separation may extend into the cemented surface by more than 1 mm, and the sum of those larger than 1/2 mm may not exceed 10 percent of the perimeter. 3.6.2
Temperature and humidity behaviour of cemented parts, coatings, blackening, resolution, filter parallelism, polished surfaces, then lenses, prisms and mirrors, reticles, wedges and windows in turn. The numbers it sets.
Systems procured unassembled are grouped per the optical diagram; assembled systems follow the instrument drawing. Defects not otherwise covered that will not impair performance are permissible.
Under 3.7, cemented components exposed to −80 ± 2 °F and +160 ± 2 °F must not develop "feathering" — the physical change in cement causing it to lose adhesion and develop into a feather-like pattern — nor show separation or softening of cement; any increase in edge separation is cause for rejection. 3.7.1, 6.3.3
Limiting sizes of surface defects are designated on the drawings by two numbers referring to two graded sets of surface quality standards per Drawing C7641866. The first refers to scratches, the second to digs. 3.5.1.1
A scratch is defined as any marking or tearing of the surface. 6.3.1 The number refers to a graded set of standards per Drawing C7641866 — unlike a dig number, the specification gives it no value in millimetres. 3.5.1.1
Four scratch types are named:
Dig numbers are the actual diameters of defects allowed, specified in units of 1/100 mm. For an irregularly shaped dig the diameter is the average of the maximum length and the maximum width. 3.5.3.1
A dig is a small rough spot on the polished surface similar to pits in appearance — generally residuals of subsurface damage caused by grinding that did not polish out, or bubbles that open up. 6.3.2
The combined length of maximum size scratches on each surface shall not exceed one quarter the diameter of that element.
Where a maximum size scratch is present, the sum of the products of the scratch numbers times the ratio of their length to the diameter of the element or appropriate zone shall not exceed one half the maximum scratch number. Where none is present, that sum shall not exceed the maximum scratch number.
Surface quality outside the free aperture of any element shall be considered 80-50, unless otherwise required.
One maximum size dig per each 20 mm of diameter or fraction thereof on any single optical surface. The sum of the diameters of all digs, as estimated by the inspector, shall not exceed twice the diameter of the maximum size specified per 20 mm diameter. Digs less than 2.5 microns shall be ignored.
Scratches which do not penetrate the glass surface fall within the same limits, and shall be considered separate from the substrate scratch requirements.
Surfaces whose specified scratch qualities are 20 or better shall have no more than 4 separate scratches in any 1/4-inch diameter circular area. Does not apply to scratches smaller than number 10.
All digs on a surface whose dig quality is number 10 or smaller shall be separated edge to edge by at least 1 mm. Measurement of scattering is not required where digs larger than number 10 are allowed.
Bubbles are classed as surface digs and any inclusion is treated as a bubble; an irregular inclusion is sized as one half the sum of its maximum length and maximum width. Tolerances are identical to digs, but the bubble tolerance is in addition to the dig tolerance.
One maximum size bubble per 20 mm of light path, or fraction thereof — not per 20 mm of diameter as for digs. The sum of all bubble diameters shall not exceed twice the diameter of the maximum size specified per 20 mm diameter, for each 20 mm of light path. Where surface dig quality is 10 or smaller, bubbles follow the dig separation rule of 3.5.3.3.
The computing diameter of a shape other than circular is that of a circle of equal area. Scratches beyond the free aperture are not counted when applying the 3.5.2.1.1 summation.
A true roof surface counts as a single surface equal to the sum of the individual roof areas, except that the roof edge itself is left out of the scratch-length summation. Roof prism tolerances assume that equivalent surface is viewed from the air side.
If not specified on drawings, the limiting size of scratches or digs shall be determined from Table I and is based on the beam diameter of magnification. 3.5.5
It is the diameter, at the surface of the optic in question, of a bundle of axial rays proceeding to the observer's eye, taken from the optical data. The diameter of that bundle at the eye is taken as 3.5 mm (0.1378 inch) if the exit pupil is over 3.5 mm; if the exit pupil is smaller, the bundle at the eye equals the exit pupil.
Where the beam is smaller than the value given for focal planes and near focal planes, the defect size is set by the magnification of the eyepiece multiplied by the magnification of the erecting system. These are the lower five rows of the table, indexed by magnifying power and focal length.
A surface is split into a central and an outer zone only where the beam diameter of an axial bundle is 25 percent or less of the free aperture. The central zone is half the free aperture in width. Reticle zone sizes instead follow 3.7.11.1.
| Focal planes and near focal planes | Central zone 1/2 diameter of surface | Outer zone | ||||
|---|---|---|---|---|---|---|
| Beam diameter (mm) | Magnifying power | Focal length (mm) | Scratch | Dig | Scratch | Dig |
| Over 5 | 80 | 50 | 80 | 50 | ||
| 4–5 | 60 | 40 | 60 | 40 | ||
| 3.2–4 | 60 | 30 | 60 | 40 | ||
| 2.5–3.2 | 40 | 20 | 60 | 40 | ||
| 2.1–2.5 | 40 | 15 | 60 | 30 | ||
| 1.6–2.1 | 30 | 10 | 40 | 20 | ||
| 1.0–1.6 | 20 | 5 | 40 | 15 | ||
| 0.6–1.0 | 15 | 3 | 30 | 10 | ||
| 0.4–0.6 | 10 | 2 | 20 | 5 | ||
| 0.2–0.4 | 10 | 1 | 15 | 3 | ||
| 0.2 | 20–10 | 12.5–25 | 10 | 1 | 15 | 3 |
| 0.4 | 10–5 | 25–50 | 10 | 2 | 20 | 5 |
| 0.6 | 5–3.3 | 50–75 | 15 | 3 | 30 | 10 |
| 1.0 | 3.3–2 | 75–125 | 20 | 5 | 40 | 15 |
| 1.6 | 2–1 | 125–250 | 30 | 10 | 40 | 20 |
A dig number is a diameter in units of 1/100 mm, so it converts directly. Every dig value Table I uses: 3.5.3.1
| Dig number | Diameter | Diameter |
|---|---|---|
| 50 | 0.50 mm | 500 µm |
| 40 | 0.40 mm | 400 µm |
| 30 | 0.30 mm | 300 µm |
| 20 | 0.20 mm | 200 µm |
| 15 | 0.15 mm | 150 µm |
| 10 | 0.10 mm | 100 µm |
| 5 | 0.05 mm | 50 µm |
| 3 | 0.03 mm | 30 µm |
| 2 | 0.02 mm | 20 µm |
| 1 | 0.01 mm | 10 µm |
A scratch number does not convert. It names a member of a graded set of comparison standards on Drawing C7641866, and the specification nowhere assigns it a width. A surface marked 60-40 therefore has a stated dig diameter of 0.40 mm and no stated scratch width at all. 3.5.1.1, 3.5.3.1
Where the drawing or instrument specification is silent, section 3.7 sets a default per element according to how far it sits from the focal plane. 3.7.9.1, 3.7.10.1
| Element | Central zone | Outer zone |
|---|---|---|
| Objectives, erectors, windows and other elements at least 15 diopters out of the focal plane | 80-50 or better | |
| Field and collective lenses | 20-5 | 40-15 |
| Center lenses of oculars | 40-15 | 40-20 |
| Eyelenses, excepting those in symmetrical eyepieces | 40-20 | 60-30 |
| Field and eyelens where identical | 20-5 | 40-15 |
| Filters between the eyelens and the exit pupil | 40-20 | 60-30 |
| Filters in front of the objective | 80-50 or better | |
| Prism surfaces at least 15 diopters out of the focal plane | 80-50 or better | |
| Prism surfaces within 5 to 15 diopters of the focal plane | 20-5 | 40-15 |
Internally located filters follow the prism requirements of 3.7.10.1, and prism surfaces within 5 diopters of the focal plane follow the reticle requirements.
Surface quality is one clause of 3.7. The rest carry numbers that are just as binding, and are the ones most often left off a drawing that says only "per MIL-PRF-13830B".
Chips that do not encroach on the free aperture are allowable, provided they do not interfere with sealing the lens in its mount. Every chip larger than 1/2 mm at its largest extremities shall be "stoned" to roughen it. The sum of the widths of chips larger than 1/2 mm, measured at the edge of the lens, shall not exceed 30 percent of the perimeter. Fractures shall be ground out, within the same limits. Stoned chips and fractures in ground faces are cause for rejection when their summed area exceeds 2 percent of the ground face or their depth exceeds 2 mm — and at any size when they interfere with the optical path, mounting or sealing.
The sum of the chip widths shall not exceed 30 percent of the length of the edge on which they occur, measured from the bevelled edge and after bevelling, not before. Chips under 1/2 mm are neither counted nor stoned; larger ones are stoned. Where the nominal shortest edge adjacent to a polished face is an inch or less, chips may encroach on the faces by 1 mm; where it exceeds 25.4 mm, by 2 mm. Fractures visible to the unaided eye on any surface or edge are not permitted.
Edges shall be trued to diameter about the optical axis by grinding, and a multi-element lens cemented and centred so each axis coincides with the others. Ocular lenses shall be concentric within 6 minutes of arc, all other lenses within 3 minutes of arc unless the drawing says otherwise. Mechanical eccentric glass overhang above 50 percent of the diameter tolerance shall be removed after centring and cementing. Optical eccentricity is the angular deviation, after refraction, of an incident ray coincident with the geometric axis of the lens.
Per the drawing tolerance. Where none is given, a filter located internally or in front of a telescope shall not exceed 1 minute of arc light deviation; a filter between the eyelens and the exit pupil shall not exceed 5 minutes of arc.
Reticle surface quality follows the focal-plane treatment of 3.5.5.3 except for the zone sizes. Where the graduation extremities fall inside it — and for reticles with horizontal and vertical lines carrying no graduations outside it — the central zone is the central area half the free aperture in width. Where graduations fall outside that area, the central zone becomes the central area three quarters of the free aperture in width. Imperfections beyond the free aperture are permitted if they do not impair the instrument.
Where the drawing gives no tolerance, reticle flat surfaces shall be parallel within 6 minutes of arc deviation of the light path. Lines shall appear of uniform width and depth with sharp intersections: line width may vary by no more than 20 percent, a line may not bow by more than half its width, and the fillet radius at an intersection shall not exceed the line width. Breaks half the line width are permitted, and on a reticle of more than 15 lines, one break per 5 lines or fraction thereof. Acid burns visible through the appropriate eyepiece are cause for rejection.
Beyond the temperature cycle of 3.7.1, cemented components exposed to +130 ± 2 °F at 95 percent minimum relative humidity, and then to −80 ± 2 °F and +160 ± 2 °F, shall not develop feathering, separation or softening of cement.
Resolution shall be tested on every objective, collective, erector, eyepiece, mirror, wedge, window, filter, prism and prism assembly. Polished surfaces shall show no evidence of grayness or stain. Wedge and window surface quality follows the lens rule of 3.7.9.1, which is why an unmarked window defaults to 80-50.
For an assembled system, 3.8.2.2 adds an alignment requirement that no element-level number covers: the exit pupil viewed on the optical axis shall have a minor diameter not less than 90 percent of its major diameter, and shall be concentric with the exit free aperture within 10 percent of that aperture when viewed from about two feet from the eyelens.
Unless otherwise specified, the supplier is responsible for the performance of all inspection requirements, and may use its own facilities or any commercial laboratory acceptable to the Government, which reserves the right to perform any inspection itself. 4.1
The element shall be viewed against a ground glass or opal surface illuminated from behind by a 40 watt incandescent or 15 watt cool white fluorescent lamp approximately 3 inches from the glass. Two or more opaque horizontal bars occupying approximately 1/2 the area of the glass shall be placed in front of and in contact with the glass.
The light through ground glass from a 40 watt incandescent or 15 watt cool white fluorescent lamp shall be passed through the element. Defects are observed by light scattered from the surface while viewing it at approximately 90 degrees to the path of the beam against a dark background.
Section 4.2 also sets procedures for temperature and humidity tests, resolution, concentricity of lenses, deviation of prisms, parallelism, and the inspection of optical systems both unassembled and assembled. Scratch and dig sizes are established by comparison against the Surface Quality Standards for Optical Elements, Drawing C7641866, rather than by instrumented measurement of the scratch itself. 4.2.3–4.2.10, 2.2, C.4.5.3.4
Outside the specification. Where a drawing needs surface imperfections graded some other way, the separate standard is ISO 10110-7:2017, Optics and photonics — Preparation of drawings for optical elements and systems — Part 7: Surface imperfections. Its scope allows permissible imperfections to be specified either by the area they affect or, alternatively, by their visibility — a dimensional method and a visibility method, side by side. MIL-PRF-13830B does not mention ISO 10110 and defines no conversion to it. ISO 10110-7:2017, clause 1 Worth knowing as well: the words "laser" and "roughness" do not appear anywhere in MIL-PRF-13830B. It was written for fire control optics, so laser-induced damage threshold and surface roughness must be specified separately.
Tighter surface quality costs polishing time and lowers yield, so the grade should follow the application. These are the grades that appear on EKSMA Optics parts.
Table I ties the tighter grades to smaller beam diameters. 80-50 is also the value the specification applies outside the free aperture of every element. 3.5.2.3
Performance Specification — Optical Components for Fire Control Instruments; General Specification Governing the Manufacture, Assembly, and Inspection of.
Intended use, per section 6.1: sights, telescopes, periscopes and range finders — as individual elements, partial or complete systems, assembled or unassembled.
Distribution Statement A — approved for public release; distribution is unlimited. Reproduced here unmodified.
Open the PDFAnswered from the specification, with the clause named. Where the answer is not in the specification, that is said plainly.
The first number is the scratch grade, the second the dig grade. 40-20 means scratches up to the number 40 comparison standard on Drawing C7641866, and digs up to 0.20 mm in diameter. Lower is tighter. 3.5.1.1, 3.5.3.1
No. This is the most common misreading of the specification. Scratch numbers refer to two graded sets of surface quality standards, viewed under the conditions of 4.2.2 and matched by eye. MIL-PRF-13830B assigns them no dimension anywhere. Only dig numbers are diameters. 3.5.1.1, 4.2.2
Outside the free aperture of any element, 80-50. 3.5.2.3 Inside it, the default depends on the element and how far it sits from the focal plane — 80-50 for objectives, erectors and windows at least 15 diopters out, down to 20-5 in the central zone for field and collective lenses. The defaults by element type lists all of them. 3.7.9.1, 3.7.10.1
They answer the same question differently. MIL-PRF-13830B is comparison-based: the inspector matches the part against the physical standards of Drawing C7641866. ISO 10110-7:2017 offers two methods instead — one grading an imperfection by the area it affects, the other by its visibility — and its third edition added a notation for quoting a scratch-and-dig figure for cosmetic imperfections. MIL-PRF-13830B does not mention ISO 10110 and defines no conversion to it, so a drawing calling out both is calling out two independent criteria.
No. The words "laser" and "roughness" do not appear anywhere in the document. It was written in 1997 for fire control optics — sights, telescopes, periscopes and range finders. Laser-induced damage threshold, surface roughness, surface figure and transmitted wavefront all have to be specified separately. 6.1
Revision B, dated 9 January 1997, superseding MIL-O-13830A of 11 September 1963. Because the extent of the changes was so large, the revision carries no marginal change notations. The PDF on this page is that document, unmodified. 6.7
They are classed as surface digs and share the size tolerances, but the bubble allowance is in addition to the dig allowance, and bubbles are counted per 20 mm of light path rather than per 20 mm of diameter. Any inclusion in the glass is treated as a bubble. 3.5.4, 3.5.4.1
By eye, under two lighting methods. Method 1 views the element against illuminated ground glass with opaque bars across roughly half of it; method 2 passes light through the element and observes scatter at about 90 degrees against a dark background. Both use a 40 watt incandescent or 15 watt cool white fluorescent lamp. 4.2.2.1, 4.2.2.2
Send the target spec and we'll tell you what is achievable, what it costs, and where a tighter number is necessary.
