The optical windows used on Princeton Instruments standard product line are some of the highest quality available. Still, for some applications, optional windows may be required.

UV/AR

NIR/AR
MG/FL
VIS/AR

Below is a discussion of the windows found on Princeton Instruments camera systems and what options are avail able for special customer requirements. Percentage of light reflected at each surface for various window coatings, including UV/AR, VIS/AR, NIR/AR, MgFl, and uncoated windows. Note that some coatings are worse than an uncoated window outside their wavelength range. Princeton Instruments cameras are provided with only one single vacuum window on the camera nose. This is required to maintain a vacuum atmosphere inside the camera so that there is no condensation when the CCD array is cooled. Window Material Princeton Instruments uses Grade 1 quartz windows in most of its cameras. This provides excellent transmission over the whole spectrum from 190 to 1100 nm. For operation at other wavelengths, such as vacuum ultraviolet, windows can be provided that are made of other materials such as MgFl. Window Defects Princeton Instruments uses the high est grade surface finish available for its windows. This minimizes the number and size of pits, scratches, and other defects to a point that they cannot normally be detected, even with a sensitive CCD array. Under some illumination conditions however, even minute defects can become visible. These conditions include very high f/# optics, parallel light and (most difficult) coherent light. Under these circumstances, some nonuniformity may be detected in a flatfield image. When necessary, flatfield correction soft ware can generally remove these low level artifacts from digitized images. Very small dirt particles from the atmosphere can become attached to the windows during use, and these can appear as small shadow airy disks in the image.

Antireflection (A/R) Coatings
Each time that light passes through an abrupt change in refractive index, some fraction of it is reflected. With uncoated quartz windows, about 3.5% of the light is reflected at each surface. The first step to take in minimizing reflections is to add antireflection coatings to input window. Princeton Instruments offers four different A/R coating options: three multilayer coatings optimized for relatively narrow spectral ranges, and one single layer coating which reduces reflections over a broad spectral range. The percent reflection per surface of each of these coatings is graphed in the figure on the previous page. Note that while each of the multilayer coatings achieves very low reflection in its intended spectral range, the amount of reflection is actually worse than no coating at all when operating outside the intended range. In addition to high reflection, these coatings can have high absorption outside their intended ranges. The graph presented on this page shows the percent of light transmitted by the multilayer coatings intended for visible and NIR ranges, when used in the blue UV. Transmission drops below 90% by 380 nm and to near zero by about 260 nm. While these coatings provide a significant in crease in optical throughput and contrast for most applications, customers who expect to work at many different wavelengths should be cautious when ordering a multilayer A/R coating, and should also consider the broadband single layer coating. Lastly, it should be mentioned that any coating applied to an optical surface is another opportunity for microscopic defects to be added. In addition, the materials used to form A/R coatings are generally mechanically softer than quartz, so they will be more subject to damage if they are not handled carefully (including periodic cleaning).
 

Percentage of light reflected at each surface for various window coatings, including UV/AR ,VIS/AR, NIR/AR, MgFl, and uncoated windows. Note that some coatings are worse than an uncoated window outside their wavelength range.

Special Problems with Coherent Light
The two surfaces of the window are very close to parallel. This makes each window act as an imperfect etalon if used with coherent light. This can lead to interference fringes appearing in an image. While the magnitude of these fringes will be fairly small (particularly with an optimized A/R coating), the high sensitivity and dynamic range of a cooled CCD detector can often detect them. To spoil this effect, Princeton Instruments can provide cameras with windows that have a slight wedge shape. In addition, we can have custom A/R coatings applied or even use customer provided windows. If you are considering a camera for imaging of coherent light, contact one of our optical specialists to discuss an optimum design for your application.