Electronic measuring technique to simplify testing of mirrors

Electronic measuring technique to simplify testing of mirrors H. KAHN,

J. GROOT,

A. BELT2

A measurement system is described in which optical and mechanical measurements of parts are performed automatically. Product inspection time is reduced tenfold. The automatic feature removes operator fatigue and judgement calls in marginal areas. Measurement repeatability is substantially improved.

KEYWORDS:

optics,automatic

testing, mirrors

Introduction It is easier, faster and more efficient to test multifaceted mirrors by a new, automated method. Inspection can be cut to one-tenth the time needed with earlier manual methods. Operator fatigue and judgement calls in marginal areas are eliminated. Other advantages are : 1.

Consistency of data: The automated measurements provide the same data on marginal components while results of manual measurements fluctuate.

2.

Faster data collection: Automatic printout can be accomplished in 5 to 10 min as compared to the 30 to 50 min needed for manual records.

3.

Faster curve plotting: Charts and other graphics that require 2 h by manual methods can be produced in 5 min by the automated system.

4.

Reduced operator training: Learning to use the automatic system requires approximately 1 h. Learning manual methods requires from 3 to 18 months, depending upon the complexity of the components and instrumentation.

5.

Simplified engineering changes: Program modification can be accomplished in 1 to 4 h for the automated system. An indeterminate amount of time is needed for manual methods, and new instruments often have to be acquired.

of the reflected beam to the reference beam (which is reflected from a known mirror). Any laser drift is thereby compensated. Other specifications such as adhesion, abrasion, surface quality, cosmetics, etc are checked manually on a small sample basis. The prior manual test method consisted of screening for low reflectance (a major problem for this mirror) by using a chart recorder. A typical plot is shown in Fig. 1. Any dips in the curve indicate a low reflectance area. While the test did not have sufficient accuracy to indicate a marginal mirror its function was to screen gross defects and by selection save time in performing the remaining tests. Mirrors passing this test would then be visually checked on a microscope as in Fig. 2. Figure 3 shows the general optics layout. Fig. 4 is a photograph of the machine. To the right of Jitter I (machine name due to mils of jitter in flatness measurement) are

This tester was designed for an eighteen-facet mirror. However, modifying certain fixturing can adapt the philosophy to testing of other mirrors. The polygon mirror tester is flexible in that repositioning various components under program control allows much of the optics to be used for different test parameters. Data on physical dimensions such as radius, pitch angle and flatness are derived by measuring the time interval between pulses. Optical properties such as absolute reflectance, scratch and dig (as it affects reflectance) are determined from the ratio The authors are at International Business Machines Corporation, General Products Division, San Jose, California, USA. Received 17 March 1982.

0030-3992/82/060303-05/$03.00 OPTICS AND LASER TECHNOLOGY.

DECEMBER

Fig. 1

Reflectance

plot of 18 facets

0 1982 Butterworth 81 Co (Publishers) Ltd 1982

303

facets should exceed 0.2 fringe. All facets should be perpendicular to DU * 0.1” and within 0.05’ of each other. Instrument

incompatibility

The computer communicates with the instruments via an interface bus. It also sends control signals to lamps and the stepping-motor controller. While the digital voltmeter could easily handle the data rate from the 18 facet mirror spinning at 2000 rpm, the counter could not. We found that when the counter used programmed trigger levels with an index pulse derived from a shaft angle encoder, we only measured the odd facets. The specifications on the counter indicated that the SO0 readings s -’ were exceeded. The solution was a delayed index. The selection of index is under program control and permits reading of the even facets. Fig. 2

Manual mirror

inspection

The following program steps are required: To initiate counter inhibit for odd facets: 0: wrt 704, ‘0000 00003’ wrt 704, ‘0000 00002’ t(MSD 8-2) = 1 and (MSD 8 -1) = 1 To initiate counter inhibit for even facets [index delayed] : 0: wrt 704, ‘0000 00001’; wrt 704, ‘0000 00000’ T_(MSD 8-2) =O and (MSD 8-l ) = 1

50% beom splitter,

Two posItIon mirror

DVM trigger pulses for reflectance and scratch/dig tests

I

The trigger pulses start with the first facet after index and use the following program steps: scanningmrror, y Vertical Slli \\

0: wrt 704, ‘00000000 1’; wrt 704, ‘000000000’

directIon

The ‘1’ in the ninth character is known as MSD 8-l. It is present inside the tester and is 1abelled“Enable ‘Home’ Detect .” Enable ‘Home’ Detect, when = ‘1’ (+5 V), allows the ‘Home’ switch to stop the stepping-motor controller.

Reference getector 2

-

w Detector

5mW laser

Fig. 3

8% beak splitter

Polorinng

This ‘Home Detect’ also connects to counter inhibit and the digital voltmeter trigger circuitry. When plus, it resets

op1\cs

HeNe

General optics layout

the test indicator lights which show which test is in progress. The test results are shown by the pass/fail indicator lights. Physical dimensions spection area. Description

were checked in the mechanical

of functional-mirror

in-

test

The mirror is tested using a HeNe laser (6328 i) with a beam size of 0.05 mm x 3.56 mm (0.002 in x 0.140 in) at the mirror surface. The long axis is parallel to the long axis of the clear aperture. The scan angle is 20” with the electric vector parallel to the plane of incidence. Reflectance should be 94% or greater gbsolute, with facet flatness 0.4 fringe maximum at 6328 A. No two adjacent

304

Fig. 4

Polygon mirror testing

OPTICS AND LASER TECHNOLOGY.

DECEMBER

1982

Home

detect

some percentage of the beam will be scattered out of the system by a scratch or dig. This loss of light is detected by detector 1. The scanning mirror is positioned at one end of the rotating mirror facet to begin the test and is then rotated incrementally in the vertical plane until the entire height of the rotating mirror is scanned. When the height of that path is finished the scanning mirror is moved horizontally to a new area of the facet. The vertical process is repeated, the cycle being continued until the entire facet is scanned.

9

Index (odd or even)

I

Counter

bit

Counter ‘2’ bit ‘T and 2’ Trigger DigItal

-: -: --l

I

I

pulses voltmeter trigger

No standard (reference) mirror is required for this test, since the specification requires that the highest reflectance be assigned a 100% value. Each reading is checked to ensure that no reflectance level is below the 6% tolerance and that no change in reflectance on adjacent facets exceeds 3% as with the functional mirror test specification.

nnnnrlw I8 negotlvegoing trigger pulses starting with first facet after second Index

1

Fig. 5 Pulse sequence, which controls the stepper motor, relation to the trigger pulses

Mirror facet flatness is tested with the optics positioned as in Fig. 8. The laser beam is divided into two with the straight through beam fixed and focused on the edge of the mirror clear aperture. The second beam can be scanned along the facet at several points. These beams are brought together and focused on detector 1. The beams are separated by about 0.75 mm (30 mils) at the detector. As the beams are scanned across a slit (mounted in front of the detector) the fixed beam scans first and triggers a counter which then measures the time taken before the second beam crosses the slit. This time is measured for all 18 facets and the difference in time between both beams can be calibrated in mils of jitter at the print plane.

in

II Light shield Cylinder lens

Lens

N

Mirror

Cahbrated

Calibration is obtained by knowing the physical distance between the two beams, the time interval taken for the two beams to cross the detector, and the speed of the rotating mirror. Absolute flatness can be determined by this test, but the best indication of a good mirror is in terms of mils of jitter. This is the difference in flatness of all the facets, measured in terms of slope variation, Pitch angle test (pyramidal error) is performed with the optics positioned as shown in Fig. 9. One of the beams is reflected from the facet and brought to a focus on detector

Detector

Detector

I

L Fig. 6

Reflectance

test

(clear) the 74192 BCD counter. When removed (goes minus = 0 V) the counter is allowed to ‘count’ the next three ‘index’ pulses. This occurs as shown in Fig. 5. Tests performed In the automatic mode the following tests are performed under program control: Absolute reflectance measurement is made with the optics positioned as in Fig. 6. Calibration of the system is performed daily. A mirror of known reflectance is placed on the spindle in place of the rotating mirror. This mirror is used in conjunction with the lower mirror to check the alignment of the lower mirror; the lower mirror is used to compensate for laser drift. Scratch and dig measurement is made with the optics positioned as in Fig. 7. This test is based on the fact that

OPTICS AN0

LASER TECHNOLOGY.

DECEMBER

1982

Reference detector

Fig. 7

Scratch and dig measurement

305

_.

Detector

the beam splitter; one beam being reflected from the facet when the facet is perpendicular to the beam, the other reflected from the same facet at 40”. Each beam is reflected to a different detector and scanned across different slits. The time interval is measured between these two reflected beams by the counter. The time difference between all 18 facets is the difference in radius. The accuracy of this test is about +O.005 mm (*O .0002 in).

I

dI-slit

”

Focwng lens I

These tests simulate the actual operation of the mirror in its application as opposed to a series of manual measurements designed to determine the suitability of the mirror in the application. A description of the manual measurements follows for comparison with the automatic mode. Manual measurements Pitch angZe(pyramidal error) consisted of a laser beam reflected from the mirror by a target 7.6 m (25 feet) distant to check the angle tolerance of 0.05” between facets.

Reference detector

Fig. 8

Flatness

Flatness was tested by a Twyman-Green interferometer where the reference mirror and beam splitter were both of h/20 quality. The tight tolerance of the polygon mirror called for extreme care in set-up. Photographs were taken of the fringes for vendor correlation.

test

Scratch and dig were measured in comparison with Davis standards. This involved operator judgement since Davis standards consist of scratches and digs on clear glass which the operator compares to a metal mirror.

Q

Focusing

lens

Reflectance was measured by comparing the raw laser beam to a reference detector and the reflected beam from the mirror. The mirror was rotated by hand. The same principle is used in the automatic tester except the mirror is rotated at 2000 rpm.

stortdetector

stopdetecior

Reference detector

Fig. 9

Pitch angle test

2. Detector 2 has two slits at 30’ to each other in front of it. As the mirror rotates, two pulses are produced with a time interval between them. As the pitch angle varies, the spot will move up or down along the slits, causing two pulses. The time between pulses will vary according to the difference of the angle change between facets. The absolute pitch angle can be measured by noting the difference in time between pulses and comparing these to the calibration mirror. The change in pitch angle can be determined by measuring the change in time around this calibrated time change.

AT

= Calibrated

Mirror radius rest is performed by positioning the optics as shown in Fig. 10. The incoming beam is split into two by

Fig. 10

306

OPTICS AND LASER TECHNOLOGY.

radius

Mirror radius test

DECEMBER

1982

Conclusions

3.

A yield increase may be expected since the functional test allows defects which will not affect the machine’s performance to pass. The manual inspection method reflects visible defects.

4.

Most technicians ing.

As stated previously, the tester performs a functional test in that it simulates the actual mirror operation in a machine. There are several desirable features of this test: 1.

The tester makes the defect call consistently.

2.

A defect record may be generated which represents what the photoconductor (machine application) would actually see.

OPTICS AND LASER TECHNOLOGY.

DECEMBER

1982

may run the test with minimal train-

The program-controlled system allows specification via programming with no hardware change.

changes

307

Illustrations: notes for authors Line illustrations

0.3 Rotring Two

1. Articles

may be published

are supplied

to the required

should

not be deterred

trations

2.

should

or paper.

lar stencils 203/2.5)

submitting

articles

be drawn

For lettering,

standards,

authors if their

as they

should

in black

should

be used (template

word

only

are not available, only

standard

can be

Leroy

the following

Authors

symbols

with

an initial

(see specimen).

lettering

and left for our studio

should

and

case (small)

could

in addi-

be printed

in

are asked to use a selection

on graphs,

of

since these are already

available

to the printer:

Authors

are asked to use the minimum

be drawn

to insert

Illustrations

tive matter points

capital

etc by their

including

size should

lettering.

not exceed

to either

172 mm or 344

The maximum

symbols

will

height

at

Black

and white

glossy

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possible,

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in soft pencil

number.

Two

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in the printed

required

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lines should

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version. be drawn

of descrip-

and place descriptive

Scale grids should

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with

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A scale, where

(if not available,

may be acceptable) and should

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or included

where on

name and the figure

of photographs appropriate,

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should

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caption.

in

and symThe thickwith

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unmounted

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amount

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Photographs 5.

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in soft

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colour should

on graphs

the caption.

If suitable

style.

mm width,

4.

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or simi-

Dimensions

drawn

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ink on these copies. illus-

size Nos. 203/3.5

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3-

>-

;-

2.0 1.5 Charge of material

308

OPTICS

AND

2.5 Cg7

LASER

TECHNOLOGY.

DECEMBER

1982