Re: Measurement of plastic optics

Greetings:

From your note I assume that you are interested in measuring the 
characteristics of plastic lenses and not the lenses in mounting  hardware 
etc.  
I checked for flare in a dozen optical engibeering  books in our library 
including the “Handbook of Optics” sponsored by the Optical Society of 
America (OSA) and edited by Michael Bass of Univ. of Central Florida, 
McGraw-Hill, 1995. While the handbook discussed the causes and prevention 
of flare spots (i.e. chipped lens rims, surface imperfections and 
scattering) and sp-ecifications for optics there were no references to the 
subject of flare except in the following books.

In “The Principles Of Optics”, A. C. Hardy, F. H. Perrin,McGraw-Hill 1932  
flare produced from multiple reflections within  photographic objective 
lenses is discussed.  The words in () are my addition QUOTE: “ In a simple 
lens containing but two 
surfaces, for example, approximately 4 percent of the light that is 
reflected at the second surface is redirected by the first surface to the 
(film) plate. The curvature of the surface may be such that an image of the 
object plane is actually formed near the focal plane, and hence every 
bright area of the object will give rise to a flare spot. On the other 
hand, these images may be formed so far from the focal plane that the 
reflected light is spread diffusely over the entire area of the (film) 
plate. The effect in the latter case is to degrade the contrast slightly, 
but this usually of far less consequence that a series of flare spots, or 
ghosts as they are sometimes called. 
    “A lens can be tested for flare by focusing it on a distant light (a 
point source) at night. If flare exists, secondary images of the light will 
be visible at various points on the (viewing) ground glass, and frequently 
images of  the diaphragm (lens stop) will appear. Of course, the more 
elements there are in an objective, the greater the chance for flare, 
unless the elements are cemented. The flare is usually less pronounced when 
the lens is used at full aperture.” END QUOTE

In “Optical System Design”, Rudolph Kingslake, Academic Press, 1983 .under 
the topic “Stray Light in Lenses” is the following QUOTE: 
“Occasionally, when a lens is stopped down to a very small aperture, an 
image of the iris diaphragm is formed by reflection in the middle of the 
picture. This image may be in focus, or it may be so out of focus that only 
a vague patch of light is formed. This is generally called a flare spot.”
   If the stray light does not form a recognizable ghost image or flare 
spots, it is generally refered to as veiling glare. This has the effect of 
lowering the overall contrast of the image, It can be measured by forming 
an image of the inside of a light box with a black absorbent disk in the 
middle of the opposite wall. A comparison of the illuminance in the image 
of the disk with that of the light background provides a measure of the 
amount of veiling glare present.”  END QUOTE. The chapter then goes on to 
discuss lens coating and mount design to eliminate flare which is 
extensivly discussed in the OSA Handbook.

   Today at our laboratory we use laser light passing through optical 
fibers to produce point sources of light for testing optics. If you have 
enough sensitivity, single mode fiber gives the optimum point source only 
one wavelength in diameter. We also 
use optical  fiber coupled to an optical detector or power meter to probe  
image planes in three dimensions.. The flexible fibers and their large 
effective numerical aperture are excellent tools  for probing and measuring 
optical systems.  

   I would measure your lenses for  flare by putting a small opaque disk on 
a glass diffuser slide at the 2F point of the lens being tested and 
illuminate it from the back with white light or the diverging laser light 
from an optical fiber. In the 2F focal plane at the other  side of the lens 
you can then probe the image of the disk and measure the contrast between 
the dark disk image and the surrounding  light to get a measure of veiling 
glare (diffuse flare). You can then remove the diffuser slide and disk and 
observe the quality of the image of the fiber point source at 2F. The focal 
plane at 2F and depth of focus can then be probed and mapped looking for 
flare spots. The use of a flexible fiber illuminator and pick up 
probe also enables off axis imaging to be tested and it’s characteristics  
probed and measured for flare effects.

Often at our lab we attach our fiber pick up  probes to the pen bar on a 
Hewlitt Packard X-Y recorder and have the recorder automatically scan and 
record a series of light profiles in the focal region. In this experimental 
set up, the plots of  the focal plane optical power distribution  are in 
full scale (1 to 1).

A red, green or blue laser can be used to measure flare at two different 
wavelengths for possible chromatic effects.
 

Best regards, your Mad Scientist
Adrian Popa