The PLab 3.0 Spectrometer is an entry-level device which allows viewing a visual-light spectrum. However, in keeping costs minimal for the user, some of the materials and construction techniques do not provide sufficient mechanical rigidity. This note describes a few simple, low-cost changes to substantial improve mechanical stability and improve measurement repeatability. Hopefully, PLab Kits can adopt and make this design available.
A spectrometer is an optical device which means critical dimensions are affected by nanometer mechanical changes. What this means is that results are easily affected by vibration, twisting, pressure and even handling. One of the primary causes for this as an issue for the PLab 3.0 (as of Sept '16) is that of the optical path. Vers 3.0 is made from paper, wood, velcro and tape. The optical path follows from the source of light, to the slit (which is mounted to the housing) to the DVD diffraction grating (which is mounted via velcro) to the camera (which is on separate velcro) and the wood base is not mechanically secured to the paper housing which holds the slit. Essentially, each component is free to move separately from each other component. Compare this with an optics lab where all components are mounted on a slab of granite. While using granite is obviously impractical, the PLab 3.0 design can be improved using very simple means.
The design shown here is only a mockup, but it is based on a real design / build of a V3 prototype shown in these two links:
Construction of those prototypes is more involved than the mockup of this upgrade design but the underlying principles are the same -- to improve stability.
The design goals of this mockup were 1) improve stability, 2) simplicity, 3) low-cost parts, 4) utilize the existing paper cover (it's not very stable but it is relatively light-tight) and 5) allow the upgrade as a retrofit. The entire design, cut/sand parts and glue assembly only took 30 minutes to complete so building a retrofit should not be very difficult.
The mockup shows the same basic form-factor as the PLab-3 device such that it should fit inside the same paper cover. However, note the important changes where the slit mount, DVD and camera are ALL part of the same mechanical assembly where all component mountings are GLUED to each other for rigidity.
The slit mount allows for several options, including just taping the photo-film slit to a black paper "carrier"; which is then double-stick taped to the wood slit-mount. This is not the only option, but having the slit-mount glued to the base is important for stability.
[ Note that there are a total of five (5) new wood pieces. All are cut from 0.5 x 0.5 inch stock; four (4) of these are 2-in long but one (1) is only 1.5-in long as it sits across the width of the base (to align the two verticals of the slit-mount and for strength). ]
Note that the same wood pieces provide a mounting surface to double-stick tape the DVD in front of the camera lens. Yes, the DVD is placed at the same angle as the camera lens. See the following for observations on diffraction angles:
Note that with a camera height of about 1/2-in, using 1/2-in square supports might be close tolerances for mounting the DVD. However, the addition of double-stick tape under the DVD would likely provide sufficient clearance. The exact distance between the camera and the DVD is not critical -- it should just be close and it should not move once it is positioned.
[ Note: The size of the 'square' parts is close to 0.5-in but the "ideal" value is set by the "height" of the camera lens after the camera is mounted. The dimension should be the same as the camera lens "height" or slightly higher -- as the double-stick tape to hold the DVD will add a small clearance. If, the mounted camera height were 0.5-in then the 'square' support dimension would be 0.5 -0.0 +0.125 . ]