Design Considerations for Reducing Stray Light

First, start with a diffraction grating as close to the definition of “perfect” in Section 10.1.6 as possible (easier said than done), and blaze it so that the first order efficiency E(λ,m=1) is as high as possible and the efficiencies in the other orders, E(λ,m≠1), are as low as possible. Provided other design considerations (e.g., dispersion) are met, it may be advantageous to choose a groove spacing d such that only the first and zero orders propagate; by Analysis of Optical Ray Paths this will reduce each sum in Eq. (10-2) and Eq. (10-4) to one element each (for m = 0). Use an entrance slit that is as small as possible, and an exit slit that is as narrow as possible (without being narrower than the image of the entrance slit) and as short as possible (without reducing the signal to an unacceptably low level). Underfill the grating and all other optical components, preferably by using a beam with a Gaussian intensity distribution. This will ensure that essentially all the light incident on the grating will be diffracted according to the grating equation.

Next, design the system to contain as few optical components between the entrance slit and the exit slit (or detector element(s)), for two reasons: each optic is a source of scatter, and each optic will pass less than 100% of the light incident on it – both effects will reduce the signal-to-noise (SNR) ratio. Specify optical components with very smooth surfaces (a specification which is more important when a short wavelength is used, since scatter generally varies inversely with wavelength to some power greater than unity).

Design the optical system so that the resolution is slit-limited, rather than imaging-limited (see Spectrometer Instrumental Bandpass); this will reduce the spectral bandwidth passing through the exit slit (whose width, multiplied by the reciprocal linear dispersion, will equal the entire spectral range passing through the slit; otherwise, the imaging imperfections will allow some neighboring wavelengths outside this range to pass through as well).

The choice of mounting can also affect instrumental stray light. For example, a Czerny-Turner monochromator (with two concave mirrors; see Section Grating Monochromator Designs) will generally have lower stray light than a comparable Littrow monochromator (with a single concave mirror) since the former will allow the entrance and exit slits to be located father apart.

Make the distances between the surfaces as large as possible to take advantage of the inverse square law that governs intensity per unit area as light propagates; an underused idea is to design the optical system in three dimension rather than in a plane – this reduces the volume taken by the optical system and also removes some optics from the dispersion plane (which will reduce stray light due to reflections and multiply diffracted light).

Use order-sorting filters where necessary (or, for echelle systems, cross-dispersers). Also, the use of high-pass or low-pass filters to eliminate wavelengths emitted by the source but outside the wavelength range of the instrument, and to which the detector is sensitive, will help reduce stray light by preventing the detector from seeing these wavelengths.

It may be advantageous to make the interior walls not only highly absorbing but reflecting rather than scattering (i.e., use a glossy black paint rather than a flat black paint). The rationale for this counterintuitive suggestion is that if all unwanted light cannot be absorbed, it is better to control the direction of the remainder by reflection rather than to allow it to scatter diffusely; controlled reflections from highly-absorbing surfaces (with only a few percent of the light reflected at each surface) will quickly extinguish the unwanted light without adding to diffuse scatter. Of course, care must be taken during design to ensure that there are no direct paths (for one or two reflections) directly to the exit slit; baffles can be helpful when such direct paths are not otherwise avoidable.

Avoid grazing reflections from interior walls, since at grazing angle even materials that absorb at near-normal incidence are generally highly reflecting. Ensure that the system between the entrance slit and the detector is completely light-tight, meaning that room light cannot reach the detector, and that only light passing through the entrance slit can reach the exit slit.

Finally, hide all mounting brackets, screws, motors, etc. – anything that might scatter or reflect light. Any edges (including the slits) should be painted with a highly absorbing material; this includes the edges of baffles.

While it is always best to reduce instrumental stray light as much as possible, a lock-in detection scheme can be employed to significantly reduce the effects of instrumental stray light. The technique involves chopping (alternately blocking and unblocking) the principal diffraction order and using phase-sensitive detection to retrieve the desired signal.

A useful technique at the breadboard stage (or, if necessary, the product stage) is to operate the instrument in a dark room, replace the exit slit or detector with the eye or a camera, and look back into the instrument (taking adequate precautions if intense light is used). What other than the last optical component can be seen? Are there any obvious sources of scatter, or obvious undesirable reflections? What changes as the wavelength is scanned? Before the availability of commercial stray light analysis software, this technique was often used to determine what surfaces needed to be moved, or painted black, or hidden from “the view of the exit slit” by baffles and apertures; even today, optical systems designed with such software should be checked in this manner.