Testing - University of Colorado Boulderecee.colorado.edu/~ecen5616/WebMaterial/26 Testing.pdfTesting • Imaging Systems ... Use autocollimator to get collimation correction ... experiment.

ECE 5616Curtis

Testing

• Imaging Systems• Aberrations and Resolution

• Interferometric testing• General light measurements• Components• Wavefront Sensor – Shack Hartmann

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Basic Imaging

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Nodal Slide – find principle planes

If not at Nodal point, lens rotation results in image motion

Joseph M. Geary, “Introduction to Optical Testing”, SPIE

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Measuring lens f/#


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Measure FFL/ BFL and EFL

Use autocollimator to get collimation correction – measure BFL/FFL

Lens equation can be used to find EFLIf PP are not accurately known iterations of these measurements will converge

Warren Smith, “Modern Optical Engineering”, SPIE

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EFL by Magnification

Use know collimator lens with Point Source and then measure image size with lens to be tested.

Used with hybred diffractive and refractive systems and fully reflective systems since the PP are commonly outside the optical package itself and thus a nodal slide would run out of travel

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Relative Intensity Falloff

Integrating sphere with detector

Lens transmission is measured as a function of chief ray angle

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Aberrations and Resolution

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Field Curvature

Measure image location along the chief ray as a function of field angle

Find COLC in z

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DistortionLateral displacement in x

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Resolution

∑−=i

iii VHAAAWAR 1

US Air Force sixth root of 2 target. Series of vertical and Horizontal bars with each set of bars smaller than the predecessor by sixth root of 2. A line pair is on black bar and one white bar. The reciprocal of this width is lp/mm.If H in focus and V not then astigmatism is present. Area weighted resolution is

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MTF and OTFMTF curve

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MTF via Knife Edge Scan

Fourier Transform of LSF is MTF through origin

Freqre

spon

se


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Interferometer

Use reference surface of extremely high quality. Fringes reveal wavefront errors which are converted into Seidel aberrations or Strehl ratios or …. Need to make certain the measurement is taken over correct area.

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Using a Null Lens

Mounting is critical. Bad mounting can warp the component.

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Zygo Readout of HDS Disk

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Ronchi Test

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General light measurements

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Measuring temporal coherenceMichelson interferometer

Fringe Visibility


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Measuring Spatial Coherence


Young’s experiment

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Ellipsometer

Intensity ratio (take square of min and max gives semi major and semi minor axis of polarization ellipse. With and without quarter wave plate can determine handedness of wave (otherwise two solutions give same result).

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Ellipsometer

Perfect V polarization V polarization after reflection off of metal mirror at an angle

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Collimation

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Beam pointing stability(PSD)

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Long Term StabilityLaser Table/Setup

Determine roughly OPL difference of needed experiment. Set OPL of interferometer to this difference and make interferometer as small as possible and cover with box. Aperture should be smaller than a fringe and just a couple of fringes visible. Output of detector to strip chart recorder and leave on ~20 hours and record output. Jumps are mode-hops, gradual changes are thermal or other drift.

Once laser is stable enough, then make the interferometer as large as the load bearing part of optical table (inside legs) keep OPL difference the same. Repeat experiment. Drifts can be thermal or bad mounts, low freqs are noise through table legs (elevators, etc), high freq acoustic

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Components

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InterferenceExample Newton’s Rings

Wikipedia

The radius of the Nth Newton's bright ring is given by

rn = ((N-1/2)λR)1/2

where N is the bright ring number, R is the radius of curvature of the lens the light is passing through, and λ is the wavelength of the light passing through the glass.

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Pulfrich Refractometer

Accurate to 5 decimal places

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Surface RoughnessProfilometer

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Scattering

Red laser

Blue laser

spatialfilter

spatialfilter

PMPM

PMPM

photo

diodephoto

diode

chopper

shutterND filter

sample

M

M

M

FM W

M - mirrorFM - flipper mirrorW - wedgePM - power meter

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Scatter• Scatter power must be calibrated vs. the solid angle subsumed by

the detector. The solid angle is calculated from the diameter of the iris (I7) and the distance from the sample to I7. The solid angle is defined as

where d is the iris diameter, and r is the distance from the sample to the iris.

• The scatter function, BSDF, is defined as

where Psc is the scattered power, and Pin is the incident power on the sample. θ is the angle between the monitor photodiode and the normal of the sample, reflecting a geometric factor of the laser beam shape ‘viewed’ by the photodiode.

2

2

2r

d

s

⎟⎠⎞

⎜⎝⎛

=Ωπ

θcos1*

in

s

sc

P

P

BSDFΩ

=

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Scatter from 2 different plastic substrates

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

2.5E-02

0 10 20 30 40 50 60 70 80 90 100

Scatter Angle (degrees)

BSD

F

Substrate 1

Substrate 2

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Absorption/transmission

Need two spectrometers to cover entire region of light.

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Spectrometer• Most spectrometer consist of three parts:

– sample– A light source covering the range of interest (infrared, UV etc..)– A monochromator to discriminate wavelengths– A detector to measure the transmitted intensity through the sample

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Wavefront Sensor

Shack Hartmann

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Shack Hartmann Wavefront SensorShack-Hartmann wavefront sensor advantages:• no moving parts • The incident radiation does not have to be coherent. Can function over wide range of wavelengths• Measures of both the irradiance and phase distributions of the light. • single CCD image captures the information, reduces sensitivity to vibration and pulsed sources may be analyzed and aligned. • Measures large range of aberrations (particularly larger than what interferometers can). • Yields the PSF, MTF, Zernike wavefront decomposition (related to Siedels), beam quality parameter' and other analysis parameters can also be performed..

Disadvantages – not as accurate as interferometer, limited resolution (need to fill sensor for good readings), limited dynamic range (can saturate)

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Shack Hartmann Wavefront Sensor

The lens array creates a pattern of focal spots on the CCD. The CCD image is broken into subapertures (called Areas Of Interest or AOIs) of N x N pixels (corresponding to the region behind each lens) in which the spot position is determined: The algorithm usually used to determine the position of the focal spots on theCCD is called the first moment or centroiding. The size of the irradiance distribution on the CCD can be characterized by the second moment of the distribution. From this the wavefront can be determined once tilt and overall beam centering is aligned out.

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Shack Hartmann Wavefront Sensor

F/5 lens, rotated -15°, large F/5 lens, rotated -15°, Evenastigmatism and defocus terms subtracted

Testing - University of Colorado Boulderecee.colorado.edu/~ecen5616/WebMaterial/26 Testing.pdfTesting • Imaging Systems ... Use autocollimator to get collimation correction ... experiment.

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