Q: What can I do with an Autocollimator?An autocollimator is one of the most flexible tools in an optical engineer's tool-kit. This informaldocument describes a few representative applications.

Theory of operationIf you need to brush up on the theory of operation these two articles are a good place to start. You can find them in the technical library section at www.wellsresearch.com

What is a collimator1? What is an autocollimator2?

A representative autocollimatorBefore getting to the applications, lets take a closer look at a typical autocollimator. The photo shows a Wells Research model AC1, mounted on a simple vertical post.

The round item on the baseplate is a mirror, which will be used in several of the application exampleswhich follow.

1 http://www.wellsresearch.com/library/pdfs/collimator.pdf

2 http://www.wellsresearch.com/library/pdfs/autocollimator.pdf

LED light source

LED light source

LED light source

Tip stage

(1 of 2)

CCD camera

Lens

Beamsplitter

Target reticle

Target reticle

Slots for LED

and reticle

Figure 1 Figure 2

Example #1: Checking parallelism

The small assembly below is a real-world example. A good 2 inch diameter flat mirror is bonded to analuminum plate. Although you can't tell from the photos, the mirror is bonded to the aluminum plateby 3 small pads of RTV adhesive. When I assembled this part, it was my intention to make the face ofthe mirror perfectly parallel to the back surface of the aluminum plate. We can use the autocollimatorto measure how close I came to achieving this goal.

The screen-shots are taken from PixelScope-Pro, the software package that is included with the AC1.The target reticle is a simple cross, whose arms are 4 mm long, and only 10 um wide.

At first light, the image is not very well centered. This means that the optical axis of the autocollimatoris not perfectly perpendicular to the mirror. By adjusting the tip/tilt stages we can bring the image ofthe reticle to the center of the screen. Now the autocollimator is exactly perpendicular to the mirror.

To turn on the small blue cross in the center of the image, select the Caliper Tool and press center

Figure 3 Figure 4

Incidentally, if you want to get make the alignment as accurate as possible, just right-click the mouseto zoom in. In the photo below you can clearly see the individual pixels in the image.

To recap, here's what we've done:

Placed a mirror in front of the autocollimator Adjusted the tip/tilt stages to center the image

Now, rotate the mirror 180 degrees. (In other words, go from figure 3 to figure 4)

Wow! That's a lot of motion. Looks like I didn't do a very good job of making the mirror parallel tothe mounting plate!

In Example #2 we'll measure this angle in milli-radians.

Example #2: Make a quick measurement of angle

OK, so the mirror surface and mounting plate are not parallel. But how much is the error angle? We can use PixelScope-Pro to measure the angular change between Our goal is to measure the angulartip between figure 9 and figure 10.

Before actually measuring the angle, it's worth reviewing the difference between angular tip(what the autocollimator measures) and mechanical tip (the problem with the mirrorassembly )

As suggested in Figure 12, the angular tip is twice the mechanical tip.

The Caliper Tool is designed for quick measurements. To use it, simply:

Select the caliper tool from the toolbar Drag the two ends of the caliper to line up with the points you want to measure.

The measured angle3 shows up on the Caliper tool window.

3 Don't be confused by the Angle item on the caliper tool. This refers to the slope of the line on the screen.

Figure 11

2A

Mirror tips by angle A

Return beam is tipped by 2A

A ray from the autocollimator (cyan)

and the reflected ray (blue) are both

exactly perpendicular to mirror

A

Figure 12

TIP:

Caliper tool

Drag with mouse

Figure 13 Figure 14

Example #3: Make a more accurate measurement of angle

The caliper tool is conceptually simple, but it can be time consuming when you need a really accuratemeasurement4. Instead, you can use the 2D Feature Finder Tool which will snap onto the featureautomatically. The process is:

Select the 2D Feature Finder Tool. Drag a selection rectangle around the cross. The 2D Feature Finder will lock onto any features within the selection region, Press Set to define this location as zero

Reverse the part (from figure 3 to figure 4) Move the selection rectangle to the new location Read the X and Y location of the directly from the 2D Feature Finder window:

4 To get good accuracy you must zoom in to the pixel level, as shown in Figure 7, and then very carefully place the calipermeasuring cross.

2D Feature

Finder Tool

Drag selection region

with mouse

Figure 15 Figure 16

Figure 17Figure 18

If you want to see the results in different units, use the Optical Configuration Dialog:

Example #4: Align two machine elements

The photo below simulates a common task. Some machine element (in this case the tip stage) has to bemade parallel to another machine element (in this case the baseplate). Often the machine elements arenot reflective, so it is common to add an optical flat, or a simple piece of glass, as shown below:

Figure 19

Figure 20

Figure21

TIP:

Here's the view5 in PixelScope:

I deliberately left a little error in the horizontal direction so you could see that there are really twofeatures in figure 23.

When you get close, the Cross Section Tool can be helpful to achieve perfect overlap.

In Figure 24 the alignment looks pretty good. However, when we view the cross section, you can seethat the traces don't overlap perfectly. If the overlap were perfect the trace would be symmetrical, andFigure 25 shows a clear asymmetry.

5 The optical flat has a partially reflective coating so that the image reflected from it is brighter than the reflection fromthe glass.

Figure24 Figure25

Example #5: Use the Autocollimator as Microscope

In the photo below, an 100 mm closeup lens has been added to the autocollimator, converting it to along-working-distance microscope.

The image in figure 27 was taken with ambient office lighting. The image shows some very smallletters on the business card, printed in gray ink. You can see that the printer achieved the gray color byusing a halftone pattern.

We can use the Caliper Tool to measure the size of the grid, but before doing that, we have to tellPixelScope that we have changed the configuration of our setup. After all, we would like themeasurements to come out in microns or mm, not degrees or milli-radians!

To do this, we use the Optical Configuration Dialog. Once we change the setup, the measurementtools will read out in units of microns.

PixelScope-Pro comes pre-loaded with a few common setups, and you can create your own with thenew and edit buttons.

Figure 26 Figure 27

Figure 28 Figure 29

To avoid confusion, the pre-loaded setups are all defined in terms of angular tip, notmechanical tip. If you would prefer to see the results presented as mechanical tip, you can usethe User Defined Units setting on the Units tab. (See fig. 19)

It is also possible to remove the reticle from the autocollimator, and use the LED as a verticalilluminator.

Example #6: Check the quality of an optical element

Situation with perfectly flat mirrorConsider an autocollimator with a simple pinhole reticle. Figure 31 shows rays from the autocollimatorstriking a mirror.

In figure 31 the mirror is exactly perpendicular to the incoming rays, but this will not always be thecase, as suggested in figure 32.

If the mirror is tipped by an angle A, then the returning rays will be tipped by 2A. These rays are stillparallel to each other, so the objective lens will focus them to a single point at the CCD.

TIP:

Figure 30

TIP:

Rays return ing to autocollimator

Rays from autocollimator

Figure 31 Figure 32

Because the rays enter the autocollimator objective lens at an angle, the point where the rays convergewill be slightly displaced from the center of the CCD.

Numerical example: The displacement will be 2*A *400, where 400 is the focal length of the objective lens in mm. If A is 1 mr, then the displacementwill be 0.8 mm. Considering that the total width of the CCD is only about 5 mm this is quite a lot!

Situation where mirror is not perfectly flatFigure 33 shows the situation when the mirror is not perfectly flat.

Of course the amount of non-flatness is greatly exaggerated. For example, an inexpensive mirrormight have a spec of only 4-6 waves per inch.

Numerical example: Consider a mirror like the one shown below, where y = 5 waves, and X = 1 inch. This is not necessarily a profile you wouldexperience in practice, but it does clearly meet the flatness spec of 4-6 wave per inch you often see on inexpensive mirrors.

y is approximately 3 microns. (When flatness is specified without a specific wavelength, 632 nm is usually implied,)

Each flat area is roughly 12 mm wide, so the mirror slope is .003/12. The angle of the returning rays is double the mirrorangle, or .003/6 = milli-radian. . The angle between the two groups of relatively parallel rays is twice this, or 1 milli-radian.

On the CCD you would see an image would consist of two relatively sharp spots, each of which has moved from the center lineby 0.2 mm, or a total separation of 0.4 mm. What should be a point image is now smeared out over about 80 pixels, eventhough the mirror flatness error is only a few wavelengths of light.

Takeaway: If you want to see a perfectly sharp return image, you will need a mirror which is really very

flat. As the numerical example shows, 4-6 waves per inch is definitely not flat enough, andeven 1/4 wave will produce noticeable image smear6. If you require a really sharp return imagean absolute flatness is 1/10 wave or better will be required.

If the return image is not sharp, it's a safe bet that the mirror is not very flat.

6 I can already hear you saying But my mirror is only spec'd for wave, and it produces a razor sharp image. Fairenough, but maybe your mirror is actually a lot better than wave. Remember, the spec only says that the flatness willbe no worse than wave.

Yx

Figure 33

Figure 34 Figure 35

Example #7: Deduce the nature of the flaws in an optical element

Example 6 introduced the concept that an imperfect mirror will tend to smear out an image that wouldotherwise be sharp. The numerical example showed how to relate the local mirror profile to how therays are smeared out.

However, what about the reverse? If we are given a setup that produces a smeared image, can wededuce the nature of the flaws in the optical element? The answer is yes, and this section explainshow.

As figure 33 suggested, an non-flat mirror will cause rays to return at different angles. The objectivelens will focus each small ray bundle to a different area at the CCD depending on the angle of the raybundle.

Figures 36 and 37 suggest a clever trick that can be used to measure the local tip of the mirror:

An aperture is used to mask all but a small area of the full beam. This reduces the total amount of light,of course, so you will need to increase the camera exposure7. However, by moving the aperturearound, you can map out the local slope variations in the mirror. This is an exquisitely sensitivetechnique.

Numerical example: It is reasonable to expect the feature finder routines to be usable to 1/10 pixel if the feature is reasonably sharp. (about 0.5 micron)This corresponds to an angle of .000 5 / 400 mm, or very roughly 1 part per million.

Now consider a mirror whose slope error is 1/10 wave per 10 mm. The actual slope is .000 05 / 10, or 5 parts per million and theslope of the reflected beam will be twice that amount, or 1 part per 10^5.

The example suggests we should be able to detect slope errors as small as 1/100 wave per 10 mm. Inpractice this level of precision is hard to achieve, and requires great careAir path noise alone caneasily introduce errors far larger than 1 ppm.

A few tips: When you move the aperture around, be careful not to touch the rest of the setup.

Likewise, when you move the aperture, be careful not to put your hand under the beam.

7 If the image is very sharp, then the small aperture will cause it to become less sharp, because of diffraction. If the imagewas fuzzy to begin with, then the small aperture will probably make it appear sharper.

Small aperture or slit

Figure 36 Figure 37

Believe it or not, the heat of your hand can cause the apparent image to move around quite a bit.

We call this Air path noise.

You can use the plot vs time capability built into PixelScope to make a plot of image locationas you move the aperture around.

If you want a more precise plot, please contact Wells Research for information about our digitaldial indicator option. This will allow you to plot image location against the actual location ofthe aperture.

Example #8: How to check the focus of the autocollimator

Before each autocollimator leaves the factory, we set both the outbound and return paths to have afocus error of no more than 150 microns. An error of 150 microns may seem like a lot (and wegenerally do better) but it it corresponds to a focus error of only .001 diopter. (in other words an imagedistance of 1 km or greater8.)

The sketch below illustrates this technique. For simplicity I show only the return path, and assumethat the source is either an image at infinity, or that you are using a perfectly flat mirror to generate areturn beam. An off-axis aperture restricts the size of the ray bundle which reaches the CCD.

Two image planes are shown. The black plane represents the actual focal plane, where the rays focusto a perfect point9. The red plane is slightly out of focus. As we move away from focus, the diameterof the spot increases, and the position also shifts slightly.

Numerical example:EFL = 400 mmAperture location = 20 mm away from optical axisAverage angle of ray bundle = 20 / 400 = .05

Distance between and red and black planes = 150 micronsVertical shift of image between red and black planes = 150 * .05 = 7.5 microns

8 This calculation uses the lensmaker's formula 1/EFL = 1/L1 +1/L2 It is usually more convenient to rearrange the formula as L2 = 1/( 1/EFL -1/L1) In the example given, EFL = 400 mm, and L1 = 400.15 mm The formula gives L2= 1,067,067 mm

9 If you are an optical engineer you will realize that even though the geometric focus is a true point, the physical image is a little larger.If there is no wavefront aberration the physical image is an Airy disk whose size is roughly F Lambda, where lambda is thewavelength of illumination. In the case of most Wells Research autocollimator this is 535 nm.

Figure 38

Now imagine that we move the aperture across the objective lens over a total range of +/- 20 mm. If we plot image location vsaperture location we'd expect to see a plot like this:

The total amount of image shift would be +/- 7.5 microns. While this is a small distance, it is easily resolvable by PixelScope'sfeature finder tool.

Another example:Consider the hypothetical mirror proposed in figures 44 and 45. We would expect to see a plot ofimage location vs aperture location like the one below:

A final example:The final example is beyond the scope of Check whether the autocollimator is perfectly focused.However, if you are an optical engineer you may find it interesting.

Please again consider the setup shown in figure 38. What if the objective lens in the autocollimatorhad spherical aberration? We would expect a plot similar to the one shown in figure 43

Figure 39: Image location vs aperture

location at black plane

Figure 40: Image location vs aperture

location at red plane

Figure 41 Hypothetical mirror with

abrupt slope change

Figure 41: Image location vs aperture

location for mirror of fig 41.

Figure 42 Image location vs aperture location for system with

spherical aberration

Hopefully you won't find much SA in your autocollimator10, but you can see how this technique can beused to measure wavefront aberation in an external element.

10 The objective lenses in our autocollimators are manufactured to a spec of < 1/10 wave aberration.