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- How do I Account for Energy Losses When Performing a Sequential Ghost Analysis?
How do I Account for Energy Losses When Performing a Sequential Ghost Analysis?
- By Akash Arora
- Published 24 June 2008
- Polarization and Thin Film Coatings , Analysis Features , Frequently Asked Questions
- Unrated
The ghost focus generator tool (Tools > Miscellaneous > Ghost focus generator) automates the process of ghost image analysis and reports a wealth of useful information. A summary of ghost statistics is given in the ghost trace data text listing.
Ghost Trace Data
File : C:\Program Files\ZEMAX - Beta\Samples\Sequential\Objectives\Cooke 40 degree field.zmx
Title: A SIMPLE COOKE TRIPLET.
Units are Millimeters.
Wavelength: 0.550000 µm
Type: Double Bounces
The '****' denotes a possible internal focus
RMS is the RMS spot radius on axis at the primary wavelength.
RMS values of 0.00 indicate that an accurate RMS
could not be computed, usually due to ray errors.
This analysis may be inaccurate if the system is
non-rotationally symmetric or uses non-standard surfaces.
Ghost reflection off surface 2 then 1. (GH002001.ZMX)
Surf Marginal F/# RMS
1 5.0000E+000 5.737068 3.5355E+000
2 4.7160E+000 4.595701 3.3570E+000
1 4.3614E+000 0.990012 3.1277E+000
2 2.7155E+000 0.607262 1.9876E+000
3 -2.2309E+000 0.916280 2.6065E+000
4 -2.7766E+000 0.514753 0.0000E+000
5 -7.3909E+000 0.888038 0.0000E+000
6 -9.0530E+000 0.823473 0.0000E+000
7 -3.4681E+001 0.823473 0.0000E+000
Marginal ray height : -34.6809
Chief ray height : -24.8107
Distance to ghost pupil: -50.9613
Distance to ghost focus: -57.1176
Effective focal length : 8.2347
In many systems, it is also very important to determine the amount of energy associated with a particular ghost path. High energy ghost foci can damage optical components, create ghost images near the detector, and cause other undesirable results.
The ghost focus generator has an option to save the files that model each ghosted path.
These files are saved in the same folder as the lens file with the nomenclature GHfffsss.zmx, where fff is the first ghost surface and sss is the second ghost surface. Opening one of these files reveals that ZEMAX models ghost reflections by changing the ghosting surface(s) to a mirror and duplicating subsequent surfaces the ray would encounter in reflection.
An uncoated mirror surface is treated as a thick layer of aluminum, which reflects approximately 91.6% of incident light; you can read more about this in another knowledge base article. This is much higher than the percent of reflected energy from a standard air-glass interface (~ 4%). A quick look at the polarization transmission report (Analysis > Polarization > Transmission) will show that the total transmission is very high (>50%). This is obviously an incorrect value for a double bounce ghost in an objective lens.
In order to accurately account for the energy in the ghosted path, a coating must be applied to each mirror surface. This coating should define what material lies beyond the reflecting surface and any coatings on that surface.
When a coating is applied to a mirror, ZEMAX assumes that the last material in the stack is a semi-infinite layer of substrate material. For ghosting from a glass-air interface with a simple AR coating, such as one quarter wave of magnesium fluoride, the following syntax would need to be added to the coating section of the coating file (Tools > Coatings > Edit coating file):
COAT GHOST
MGF2 0.25
AIR 1.0
The last material defined is air, which is assumed to be the substrate. Once a suitable ghost reflector coating has been defined, the ghost focus generator allows the user to specify this coating for use during that analysis.
After applying this coating to the mirrored surfaces in our ghost file, the total transmission is a much more reasonable value.
This method can be used for surfaces that have more complex coatings and which may be immersed in a material other than air. As long as the materials are properly defined in the coating file, they may be used in any coating. For a more detailed discussion of coatings, see Chp. 20 of the ZEMAX manual.
Ghost Trace Data
File : C:\Program Files\ZEMAX - Beta\Samples\Sequential\Objectives\Cooke 40 degree field.zmx
Title: A SIMPLE COOKE TRIPLET.
Units are Millimeters.
Wavelength: 0.550000 µm
Type: Double Bounces
The '****' denotes a possible internal focus
RMS is the RMS spot radius on axis at the primary wavelength.
RMS values of 0.00 indicate that an accurate RMS
could not be computed, usually due to ray errors.
This analysis may be inaccurate if the system is
non-rotationally symmetric or uses non-standard surfaces.
Ghost reflection off surface 2 then 1. (GH002001.ZMX)
Surf Marginal F/# RMS
1 5.0000E+000 5.737068 3.5355E+000
2 4.7160E+000 4.595701 3.3570E+000
1 4.3614E+000 0.990012 3.1277E+000
2 2.7155E+000 0.607262 1.9876E+000
3 -2.2309E+000 0.916280 2.6065E+000
4 -2.7766E+000 0.514753 0.0000E+000
5 -7.3909E+000 0.888038 0.0000E+000
6 -9.0530E+000 0.823473 0.0000E+000
7 -3.4681E+001 0.823473 0.0000E+000
Marginal ray height : -34.6809
Chief ray height : -24.8107
Distance to ghost pupil: -50.9613
Distance to ghost focus: -57.1176
Effective focal length : 8.2347
In many systems, it is also very important to determine the amount of energy associated with a particular ghost path. High energy ghost foci can damage optical components, create ghost images near the detector, and cause other undesirable results.
The ghost focus generator has an option to save the files that model each ghosted path.
These files are saved in the same folder as the lens file with the nomenclature GHfffsss.zmx, where fff is the first ghost surface and sss is the second ghost surface. Opening one of these files reveals that ZEMAX models ghost reflections by changing the ghosting surface(s) to a mirror and duplicating subsequent surfaces the ray would encounter in reflection.
An uncoated mirror surface is treated as a thick layer of aluminum, which reflects approximately 91.6% of incident light; you can read more about this in another knowledge base article. This is much higher than the percent of reflected energy from a standard air-glass interface (~ 4%). A quick look at the polarization transmission report (Analysis > Polarization > Transmission) will show that the total transmission is very high (>50%). This is obviously an incorrect value for a double bounce ghost in an objective lens.
In order to accurately account for the energy in the ghosted path, a coating must be applied to each mirror surface. This coating should define what material lies beyond the reflecting surface and any coatings on that surface.
When a coating is applied to a mirror, ZEMAX assumes that the last material in the stack is a semi-infinite layer of substrate material. For ghosting from a glass-air interface with a simple AR coating, such as one quarter wave of magnesium fluoride, the following syntax would need to be added to the coating section of the coating file (Tools > Coatings > Edit coating file):
COAT GHOST
MGF2 0.25
AIR 1.0
The last material defined is air, which is assumed to be the substrate. Once a suitable ghost reflector coating has been defined, the ghost focus generator allows the user to specify this coating for use during that analysis.
After applying this coating to the mirrored surfaces in our ghost file, the total transmission is a much more reasonable value.
This method can be used for surfaces that have more complex coatings and which may be immersed in a material other than air. As long as the materials are properly defined in the coating file, they may be used in any coating. For a more detailed discussion of coatings, see Chp. 20 of the ZEMAX manual.
