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Public Lab Research note


Webcam and PVC Spectrophotometer

by CTormey | September 09, 2021 23:05 09 Sep 23:05 | #27718 | #27718

Purpose

With an increase in the need for remote laboratory equipment we have designed and tested a relatively cheap spectrophotometer for use at home or mobile laboratory setting.

Materials needed (All prices are as of 09-09-2021)

Helpful Tools

  • Dremel Tool with Plastic cutting wheel (Amazon Link)
  • Hack saw
  • File
  • Vice
  • Wire foam cutter
  • Tape measure
  • Ruler
  • Scissors
  • X-Acto knife
  • Permanent marker
  • Flashlight
  • CFL
  • Miter or chop saw

Step 1 (Diffraction Grating)

First, we will cut our diffraction grating down. It is too wide to use with the 1 in. PVC and so will be trimmed. The larger sections of cardboard on the top and bottom will be trimmed down to a few millimeters. Care should be taken not to trim it too much as it still needs to hold as rigid and flat a shape as possible. See the image below for how it should be trimmed.

Trimmed diffraction slide vs stock slide

Step 2 (Diffraction Grating Housing)

Next, we will create the diffraction grating housing using the 1 in. 45-Degree elbow piece. Painter's tape is put about 2 mm from one end of the fitting. Care should be taken to ensure it is square with the edge. Then a permanent marker is used to indicate where to cut. Use the slide that was trimmed in the previous step to help indicate how wide the cuts should go. The cuts will have to be slightly wider than the slide to account for the thickness of the PVC pipe itself.

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The housing can then be cut by hand using a saw or Dremel tool with cutting wheel. Care should be taken to keep the cutting implement square with the surface of the pipe to ensure the diffraction grating slide will be flat when inserted.

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After the initial cut try inserting the trimmed diffraction grating from Step 1 into the slot. The diffraction grating should fit as shown in the following image. If it does not, it may be necessary to clean the edges or cut a larger opening.

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Step 3 (Sample Housing)

We will be using the 4-way tee fitting to create a place to hold our sample cuvette. The only modification required to this piece is to shorten one of the 90-Degree openings to allow for easier sample insertion and removal. To do this we will again wrap a piece of painters tape flush with the end of the opening. This can be used to show where the pipe must be cut. A Dremel tool or hack saw can be used for this step, however, we found that it was easier to use a chop. A spare piece of 1-in. PVC pipe was inserted into the fitting opening designated to be cut and a thin piece of wood was shimmed against the pipe. This could then be clamped down on the saw tabled and the cut was easily and cleanly made. (see image below.)

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Step 4

The last piece needed is an approximately 3.5 in piece of 1 inch PVC pipe.

Step 5 (Painting)

In order to reduce light scattering and glare the inside of all the pieces was painted with the Rust-Oleum flat black primer. All pieces were placed on cardboard and painted and left to dry for 24 hours. It was important not to let the paint to be too thick fitting openings to allow the other PVC pieces to fit together easily. Make sure to include painting the 1 in. to 3/4 in. reducer bushing when painting as this will be used to house the webcam. (See image below)

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Step 6 (Foam Pieces)

We will use the pink foam to create two different pieces. The first is a holder for the cuvette and sample. And the second is a shim to hold the web camera in place. We used a wire foam cutter to make most of our pieces though an X-Acto knife can also be used. To create the holder a circle the size of the inner diameter of the 1 in. 4 way tee fitting is drawn. A cylinder is cut from the foam. Care should be taken to make the sides as vertical as possible, though they don't have to be perfect. It is best to err on the side of being slightly too wide to fit into the opening as the foam can be trimmed slightly and will squish a little. Then a square hole is cut in the top that is sized for the cuvette. Again it is better to be slightly smaller as pushing the cuvette can squish the foam slightly and create a tight surface. The light path can be created by cutting a hole perpendicular to the cuvette hole. We found the best results by inserting the holder first with a cuvette in. Then the cuvette could be adjusted to be in the correct orientation. The cuvette could be removed and hot needle foam cutter could be pushed through to create a light path. (see image below for final result)

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To create the camera shims a cylinder is cut similar to above but with the diameter being that of the inner diameter of the 1 in. to 3/4 in. PVC reducer bushing. The inside is then cut out to create a hallow cylinder around 4 mm thick. This is then cut in half long wise to create two half-pipes. Below is an image with these on the camera device. In the background you can see how it is sketched on the foam.

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Step 7 (Assembling the device)

Because so much of this is made by hand care must be made when assembling the device to ensure different parts line up to allow efficient transmittance of light through the sample and to the detector. We will first mount the webcam board into its housing. To do this put the two pieces of cut purple foam on the base of the camera (it looks like a square block.) You will then insert the camera into the housing. Here is where you need to "fiddle" with it a bit. The foam will help squeeze the camera into the housing to keep it secure, but you may need to adjust the foam so that the camera lens is as centered as possible. You also want the top to be as flush and square as possible, so the camera isn't cocked at a slight angle.

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The following steps will involve lining up the light source, the light entrance slit, the cuvette holder and the focusing slit. The idea is we want a clear path of the light to go from entering the device all the way through the sample and finally through a narrow slit that will focus the light onto the diffraction grating and ultimately be captured by the webcam. Because each device is hand made the location of the entrance and focusing slits may not be exactly centered. The following diagram shows you how things should line up.

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First, the entrance slit is made and it should be lined up with the holes made in the purple foam cuvette holder. The width of the entrance slit should be made out of the black cardstock and be approximately 5mm but there is room for error. The main thing is to have most of the light allowed in shine in a band on the entrance hold of the cuvette holder. This will limit the amount of scattered light that will end up at the detector. The first image below shows an example of this entrance slit. To ensure that the light is properly aligned, use a piece of white paper and see if you get a small bright spot. This will be the light coming through the cuvette holder correctly. While you will need to use the provided lamp when calibrating and experimenting it may be helpful to use a flashlight to do the alignment.

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We then need to align the focusing slit. To do this you need to cut a small round piece of the black cardstock. It is important that this piece is slightly smaller than the diameter of the pipe. This is because after it is secured to the end it should not be pulled over the sides when the pipe is fit into the end of the cuvette holder. If it does, it will cause the cardstock to bend or buckle when pushed into the cuvette holder and this will ruin the image.

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You will then need to determine where to cut the narrow slit used for focusing the image. To do this hold the pipe with the black paper on the end in approximately the place it will be when secured in the pipe. You should then use a pencil to see where the center of the circular light image leaving the cuvette holder is. You can then, using a razor blade, boxcutter or X-Acto Knife cut a very narrow slit in the spot where the majority of the light will hit it. When done you should get a fairly sharp line of light when projected onto a white piece of paper.

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You can check to see if things are lined up well by looking directly at the grating with a light source at the other end of the spectrophotometer. If you see a rainbow pattern like the following image it should be working. It may not be perfectly centered but that is ok.

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You can then insert the camera housing into the end of the diffraction grating holder. This can be rotated as needed to get the best image in the correct orientation.

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Finally, you will want to cut a strip from the black cardstock to cover the top of the diffraction grating holder to eliminate any stray light coming in. Then you can plug the camera into your laptop/computer and use a camera app to see if you see a rainbow image like the one shown below.

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3 Comments

Very interesting project!! What kind of lamp do you use and how powerful does it need to be? Do you know what spectral range it and the instrument will cover? It looks like your resolution should be better than a standard spectrometer. Have you had a chance to evaluate resolution? Thank you.

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I went over it again. Impressi ve. Are you thinking of doing fluorescence work it, too?

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Sorry to bother you. How do your spectra look? How has the instrument been standardized? Thank you. Regards.

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