Spectral Test & Calibration Station 4 Grating Turret for Wide Wavelength Range Fast f/4 Aperture Ratio, Better Througput Signal 125-mm Clear Aperture Collimated Output Built In Light Sources Reference Detector(s) The McPherson Spectral Test & Calibration Station is designed and built to measure or test spectral response at a variety of discrete wavelengths over a designated spectral range. The spectral range is determined by light sources, monochromator design, gratings, detectors and optical coatings (and materials, for systems using refractive optics.) Delivery of the discrete monochromatic light to an integrating sphere, collimator/telescope or other relay or instrument optics is determined by user requirements.
Light source energy is collected by reflective condensing optics and focused to the monochromator entrance slit. The condensing assembly houses (optional) light chopper and filter for improved spectral purity. Monochromator entrance and exit slits are adjustable in width and height. Slit width and grating (and monochromator) determine bandpass. Computer controlled monochromator sets wavelength band and scan range. Monochromator gratings are selected to be most efficient in the region of interest (eg. MWIR, LWIR, etc.) Monochromatic light is focused directly at the aperture in systems requiring maximum power and less uniformity. In systems requiring high uniformity the monochromatic light is directed to an integrating sphere then to the telescope or relay. For highest spectral purity use a double monochromator. The optical system delivers a high quality beam with user specified clear aperture to the test object. All optical elements used in this system are reflective, free from chromatic aberration and transmittance effects. Reference detectors and sample mounting considerations are determined with customer requirements for operation and data production. The monochromator, light source, target and collimator for are built into a system. This guarantees alignment and simplifies set up in the users laboratory.

The Monochromator The most popular monochromator used with these systems is the 500mm focal length, f/4 Model 205. Mount up to 4 gratings in the quad- turret for unsurpassed range capability. The large gratings operate at about f/4 guaranteeing good throughput. The 205 is responsible for selecting the specific wavelength of test and determining bandpass of light during measurement. Diffraction gratings in the monochromator determine the range and efficiency of operation. The focal length determines dispersion and combined with slit size, bandpass. This McPherson monochromator uses a sine drive mechanism to drive the gratings and provides exceptional wavelength accuracy and reproducibility, a must for test systems that need to also demonstrate test-to-test repeatability. The gratings rotate about the center of their optical surface and thereby maintain good aperture while rotating. The grating is driven by a stepper motor system (McPherson Model 789A-3.) and is computer controlled.

The entrance and exit slit sizes may be varied continuously from 0.01 to 4mm. Slits may be computer controlled or replaced with pinholes for higher reproducibility albeit less flexibility. Compare the dispersion value provided in the monochromator grating specification table with the slit width setting to explore variability of the bandpass. The limiting factor of the slit size will usually be the systems diffraction limit.The instrument allows to attain these values given sufficiently sensitive detectors, to deliver sufficient light intensity to object under test small slits such as these may not be practical.

The Collimator The collimator is designed to deliver a 4inch (100mm) diameter beam to the sample. The coupling from the monochromator to collimator is typically optimized for light intensity. The monochromator exit slit is imaged 1:1 to an aperture (or multi-aperture wheel) that feeds the collimation optics. Collimation optics normally consist of a spherical or off axis parabolic mirror combined with a fold flat. We also provide relay optics systems using spherical optics, crossed cylindrical optics and combinations of these. Talk to us about your requirement when systems are intended to match specific sensors or flight system FOV.

The direct coupling of the monochromator to the collimator provides highest light throughput. In doing so uniformity is not very high and there can be a gradient of wavelengths (within the bandpass) across the aperture. If spectral or intensity uniformity across the beam is critical to the application a small integrating sphere can be used to homogenize the light exiting the monochromator.

The collimator enclosure is equipped with robust and stable mounts for the collimating mirror, fold flat and entrance aperture(s.) A second, small, fold flat is used as a 'pick-off' mirror to divert a small portion - if possible outside the clear aperture - of the light to a detector. This is a reference detector. The signal monitored at this detector can be used for long term stability monitoring. In addition, the reference detector signal can be compared to a second detector, optionally mounted in the sample position. The ratio of the two signals provides a correction factor possibly useful in comparing results attained with 'unknown' sensors.

In the specialized version of this system pictured above, a heat seeking missile was kinematically mounted and the dome of the missile received a 5 inch diameter collimated beam from the all reflective beam collection and collimation housing. The hyperbolic optical receiver of the missile transferred the scanning monochromatic energy of the calibration system to the detectors. The previously unknown spectral and peripheral response of the missile was determined and its spectral sensitivity was analyzed. Due to the accessible sample area, many other specimens and samples can be measured.