tn108
Instrument Science Group
Royal Greenwich Observatory
Design and Use of a Novel Flat
Field Illumination Light Source
TECHNICAL NOTE 108.
Simon Tulloch. 6th Nov 1996
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Instrument Science Group
Royal Greenwich Observatory
1. Introduction.
Flat field exposures are essential for highlighting cosmetic defects on CCD chips during the
optimisation process. They are also useful for diagnosing surface contamination on UV flooded and
Passivated Platinum Coated CCDs. This contamination is clearly visible since it causes a strong
patterning in the flat field image.
Obtaining flat field images in the laboratory is not easy. A single light source positioned in front of
the CCD camera will produce a fairly flat illumination if it is positioned far enough away, however,
this will usually require a darkroom. After some experimentation with a spreadsheet model it was
found that a remarkably flat illumination profile could be obtained from the overlapping illumination
provided by four diffuse light sources. The source geometry meant that a very compact flat field
projector could be designed to fit conveniently onto the front of a CCD camera cryostat.
This document explains the geometrical principles, the mechanical design and the use of this flat field
projector.
Figure 1) The flat field projector in use
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Instrument Science Group
Royal Greenwich Observatory
2. Properties of a Diffuse Reflector.
At the heart of the flat field projector is a sand-blasted aluminium plate. This plate is illuminated by a
number of LEDs and the light scattered by it combines on the CCD to produce a very uniform
illumination pattern. Sand-blasted aluminium scatters light in a ‘Lambertian Distribution’. This is
shown graphically below in figure 2.
Figure 2) The Lambertian Distribution.
LAMBERTIAN
SCATTERING
SURFACE
The luminous intensity of the
scattered rays is proportional to COS φ
φ
If a CCD is placed below this scatterer in a plane parallel to it, as shown in figure 3, it will receive an
illumination that varies across its surface as COS4φ, where φ is the angle between the illuminating
light source, the diffusing surface and the CCD. This relationship is fairly easy to understand: the first
cosine term is due to the Lambertian Distribution, the second to the inverse square law drop off in
illumination, the third to the projected area of the scattering surface decreasing with increasing angle
and the fourth is due to the same effect in the CCD itself.
Figure 3) Illumination profile on a plane parallel to a Lambertian Scatterer.
LAMBERTIAN
SCATTERING
SURFACE
φ
Illumination (I)
CCD
I=COS4φ
Clearly a single Lambertian scattering source will not by itself provide a flat field illumination.
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Instrument Science Group
Royal Greenwich Observatory
3. Flat Field Projector Design Theory.
The flat field projector uses four light sources to illuminate the aluminium scatterer. The resultant
overlapping illumination profiles can be modelled easily using a spreadsheet. The model shows the
light sources arranged in a square pattern, the plane of this square being parallel to that of the detector.
Figure 4 shows the resultant illumination profiles for three specific geometeries. In this model the
LEDs are in a ‘star’ arrangement i.e. the sides of the square that join the light sources are parallel to the
sides of the CCD. The alternative ‘cross’ arrangement where there is a 450 offset is not optimum and
results in under-illumination of the CCD corners.
Figure 4) The Illumination profile from an array of four Lambertian scatterers radiating
downwards onto a plane.
Z
1
CCD
PLANE
1
Z=1.2
100
% of centre
Illumination
%
illumination
98
96
94
92
Z=1.2
90
88
86
84
82
80
Z=1
This geometry
provides optimum
flatness in the
central area.
100
%
of centre%
Illumination
illumination
99
98
97
Z=1
96
95
94
93
92
91
90
Z=0.8
100
%
of centre
Illumination
%
illumination
99
98
97
96
Z=0.8
95
94
93
92
91
90
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Instrument Science Group
Royal Greenwich Observatory
4. Mechanical Design of the Flat Field Projector.
The design target was to achieve a flat field accuracy of 0.25% when illuminating a 30mm square CCD
such as a Loral 2K x 2K or a TEK1024. The spreadsheet model showed that this would require a CCDdiffuser plate separation of 70mm.
The projector, shown schematically in figure 5, consists of a light-tight aluminium barrel with a
diameter of 124mm and a height of 69mm. The base of this barrel was threaded so that it could be
screwed onto the face of the camera cryostat in front of the window. This first required a cryostat
interface plate to be attached to the front of the cryostat. This plate had a central 96mm diameter
threaded aperture into which various photometry tools could be mounted. An ‘O’ ring mounted in a
groove on the lower face of this plate ensured a light tight seal. The aluminium diffuser plate was
113mm in diameter and mounted close to the top of the barrel. It was illuminated from below (i.e. from
the CCD side) by LEDs mounted in a ring. Light from the LEDs was scattered by the plate and passed
back through the centre of this ring and onto the CCD. The ring actually contained three colour groups
of LEDs so that flat field images could be taken in the ultra-violet, visible and infra-red parts of the
spectrum. Each of these groups contained four LEDs arranged equidistantly around the ring. Light from
each LED passed up through a 2mm diameter collimation tube and illuminated a 5mm diameter spot
near the periphery of the diffuser plate. Recesses on the top side of the ring allowed coloured glass
filters to be inserted at the ends of the collimation tubes. Filters were necessary to shift the peak
wavelength of the LEDs used for the ultra-violet measurements from 470nm to 390nm. For this, 10mm
diameter x 2mm thick discs of UG1 glass were used. The visible and infra-red LEDs were used unfiltered and these recesses remained empty. All the exposed inner surfaces of the projector were sandblasted and, with the exception of the diffuser plate, black anodized to reduce stray reflections. An
electrical connector on the top of the barrel made direct connections to the internal LEDs.
Figure 5) Mechanical Schematic of the Flat Field Projector
1
2
3
4
12
5
7
7
6
8
8
11
10
7. Mounting screws.
8. Light tight seal.
9. Cryostat body.
10. CCD.
11. Cryostat window.
12. LED mounting ring
1. Power connector.
2. Diffuser plate.
3. Space for filter.
4. Collimation tube.
5. LED.
6. Cryostat Interface Plate
5
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Instrument Science Group
Royal Greenwich Observatory
Figure 6) Detail of LED Mounting Ring.
5. Illumination Wavelengths.
Figure 7 shows the colours of the three LED groups used in the flat field projector. The peak
wavelengths occur at 390, 565 and 950 nm.
Figure 7) LED Spectra.
1
0.9
0.8
relative output
0.7
VIS
UV
0.6
IR
0.5
0.4
0.3
0.2
0.1
0
350
450
550
650
750
wavelength (nm)
6
850
950
1050
Instrument Science Group
Royal Greenwich Observatory
6. Performance of the Flat Field Projector
The performance of the projector matched the spreadsheet model. The flat field images obtained were
very uniform right to the image corners but exhibited a slight tilt. This tilt can be explained by
brightness differences between individual LEDs within a colour group. Figure 8 and 9 show actual
data from a TEK1024 CCD flat field image using 565 nm LEDs. The slight turn up at the edge is a
genuine effect : the QE of this CCD is higher at the edges, at least in the green part of the spectrum.
Figure 8) IRAF Surface plot of a flat field image obtained at 565 nm.
Figure 9) Cross sectional plot of previous image.
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Instrument Science Group
Royal Greenwich Observatory
The flat field projector was also used to diagnose QE degradation due to surface contamination in a
Loral Lick 3 CCD. Figures 10 and 11 show flat field images taken before and after UV flooding. It is
easy to see that the QE has been degraded in the first image even without making accurate QE
measurements.
Figure 10) A flat field image from a contaminated Loral Lick 3 CCD
Figure 11) A Flat field image from a recently UV flooded Loral Lick 3 CCD
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Instrument Science Group
Royal Greenwich Observatory
The flat field projector has also been used with the new large format EEV42 CCDs. The spreadsheet
model predicts a 4.5% drop-off in illumination at the corners of this larger (28 x 55mm) device.
Nevertheless the flat field images obtained, one of which is shown in figure 12, are still very useful.
Figure 12) IRAF surface Plot of a flat field image taken with an EEV42 Large Format CCD.
7. Future Improvements.
In its current form the flat field projector is limited in its accuracy by the brightness errors of the
LEDs. Since the LEDs in each colour group are connected in series there is no way to control the
brightness of each one individually. Greater accuracy will be achieved by using parallel connections
and tuning the brightness of each LED by a potentiometer. More uniform illumination can also be
gained by increasing the diffuser plate - CCD separation. The table below shows this for a number of
geometries. In each case it is assumed that a 30mm square CCD is used. All the dimensions will scale
linearly i.e. if a CCD measuring 60mm on a side were being used then the diffuser plate - CCD
separation would need to be exactly doubled to achieve the same flat field quality. The third column of
data is included to show the importance of rotational alignment with the CCD if optimum
performance is to be achieved.
Diffuser Plate-CCD
seperation
(mm)
35
70
140
280
N.B. Assumes CCD is 30mm square.
Maximum variation in
illumination : ‘Star’
Configuration
Maximum variation in
illumination : ‘Cross’
Configuration
5%
0.25%
0.02%
0.001%
26%
6.3%
1.5%
0.37%
8. Acknowledgements.
I am grateful to Terry Dobner of the RGO Mechanical Workshop who manufactured all the parts for
the flat field projector.
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Instrument Science Group
Royal Greenwich Observatory
APPENDICES
A. LED connections in the flat field projector.
LED 1,2,3,4
J1- 1 (UV)
J1- 2 (VIS)
J1- 3 (IR)
LED 5,6,7,8
J1- 16
LED 9,10,11,12
B. CCD Controller Interface Box.
This box allowed the LEDs in the flat field projector to be controlled from the CCD controller using
the pre-flash LED signal. In order to obtain deep images the projector will need to be switched on for
about 5 seconds.
+20V
Vin
10µF
7815
Vo
GND
330R
100nF
J3- Core
J3- Sheath
10K
SFH610a-2-X001
220R
3K
Test
BC109
2K
680R
•
•
•
27K
300K
330R
•
J1 -Core
J1 -Sheath
BC179
Front
panel
indicator
J2- 1 (UV)
J2- 2 (VIS)
J2- 3 (IR)
J2- 16
J1 is used to power another piece of equipment : the LED reference lamps.
J3 receives the ‘preflash’ signal from the CCD Controller. Details of the connection cable are
shown in appendix C.
J2 is used to connect to the flat field projector.
A front panel indicator LED shows when the circuit is active. A front panel single pole nonlatching switch allows the circuit to be activated manually.
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Instrument Science Group
Royal Greenwich Observatory
C. Cable Design
The standard cryostat to CCD controller heater cable has an RS404-468 cable mounting plug at the
controller end and an RS404-474 cable mounting socket at the cryostat end. The cable used to connect
the controller to the interface box shown in appendix B modified this design by connecting a co-axial
cable to the controller end of the cable. The sheath was connected to pin D, the core to pin C.
D. Part Numbers
The LEDs used are all available from RS and have the following part numbers:
LEDs 1,2,3,4 (UV) were Nichia NLPB500, RS Stock No. 199-6227
LEDs 5,6,7,8 (VIS) were HLMP8509, RS Stock No.865-672
LEDs 9,10,11,12 (IR) were LD274, RS Stock No. 195-669
The UG1 glass filters were obtained from UQG, Tel : Cambridge 420329
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