Re: "Colorimeter"

Spinning or oscillating prism?

How much spectral resolution could you get from a cell phone image?

How broad and how much resolution? There are sensors, eg:

CCDs are almost indestructible unless you point them at the sun. Even then they handle it much better than a human eye. Webcams are probably a lot cheaper though. If you find one of the paint firm's colour matching apps and test chart it may already do what you want or close enough.

If you just want to color match then your phone camera is dandy. There are apps used by printers and film lighting cameramen to do just that. ISTR chromlink ?

use a CD

eons ago we used some CCDs as detectors for X-ray fluorescence, some had weird formats like 1024x64 pixels so I assume they were really made for spectroscopy

If it is spinning steadily, all you need is a synchronising pulse at some point once per revolution and a wide spectrum photocell with an optical slit and a lens. Software can work out the wavelength from the rotational speed and the known characteristics of the prism. The resolution can be as coarse or as fine as you like and algorithms can work out the visual perception of line spectra (if that is what you need).

The same hardware could be used for an expensive high-resolution device or a cheap and cheerful version - the software and the time to reach a steady reading (longer integration period for lower 'noise') being the only real differences.

As ever the devil is always in the details. Identical colours but with different surface finishes can look incredibly different. Vantablack is very much like looking into the void it is quite literally blacker than black!

Any other "black" looks grey next to it.

You can trick almost any sensor. Human eye can be quite easily misled by didymium glass which is a narrowband Na-D blocking filter used to see into a bright yellow sodium flame when glassblowing.

Side effect is to produce cartoon like out of gamut colours when the brain tries to compute colours from the cones. Its apparent colour varies radically with the source of illumination.

The same property is shared with the natural gemstone Alexandrite.

You vary the exposure to avoid spillover.

or a shutter to limit the time light hits the sensor

Mount the prism on a a flywheel and spin it rapidly. The only jitter might come from errors in the timing pulse (or knackered bearings!).

One way of obtaining a jitter-free timing pulse would be to reflect a known pattern of light off the faces of the prism into the photocell; use the software to recognise it and make corrections for any long-term speed drift.

If it spins faster you can simply integrate multiple 'passes' for as long as you want until the noise is negligible. The frequency response of the photocell and head amplifier is likely to be far wider than any mechanical system needs, so the physical narrowness of the slit and the distance from the prism will set the resolution limit. .A narrow and distant slit will give higher resolution at the expense of a worse S/N ratio, which can be overcome with a longer integration time.

Yes, it has many advantages.

"Cheap and cheerful" is a slang [UK English] expression meaning a quick rough estimate or goods that aren't intended for serious long-term use.

I really wouldn't consider anything with moving parts. You can get reasonable grade replica grating for low resolution spectroscopy from the likes of Edmund scientific (intended for school labs).

UK alliterative saying. I guess it doesn't translate into USAian too well.

Figure out how much resolution you need before starting out.

A shovelware DVD at glancing incidence can resolve the absorption lines in the suns spectrum if you do it just right. You look down onto the disk with the sun at a very shallow angle to the surface. Don't look at the reflection of the sun - only at the very dispersed spectrum.

The spectrum obtained with this simple kit is impressively high resolution. It will also have various funny organic dye lines in with a modern writeable one.

You really want aluminised media for this trick.

I was assuming very fast sampling so that the presentation of each line was captured by many samples, that way the software could sort it out over a large number of repeated passes. Keep the hardware simple and let the software deal with the errors if it can be given enough data to start with.

Less developed software, lower sampling rate, slower ADC and less memory in the cheaper version. (And a garish box with "Professional" on it, to let customers know that this is the cheap and nasty version.)

That's the sort of thing. An interesting demonstration toy, rather than a laboratory instrument.

...or the transparent disc used to protect the end of a 'cake' of CDRs.

If you put your nose into the hole in the middle and shut one eye, it displays a good spectrum.. I once had a collection of eminent people all sat around a table doing this at a coffee morning in the Bath Royal Literary and Scientific Institution.

To my non-UK but Commonwealth-derived ear, "cheap and cheerful" is, well, more cheerful than DY's proposed equivalents.

I'd translate it more as "dollar-store" or "chinesium".

Cheers

Phil Hobbs

If the light levels are very high then you can get LEDs with emission profiles of about 50nm width. They work as sensors in the opposite direction with some leakage for higher energy photons. Choose them wisely and calibrate against a reference white and you might have something that is both cheap accurate and durable.

I was expecting to sweep the whole spectrum at high speed many times, then analyse the captured data. Television-type technology could easily cope with that data rate from a single photocell.

The ratio can be varied by either the user or the designer of the instrument. If greater accuracy is required, it will take longer to do both the capture and the analysis.

Some sort of reference source could be used to generate a known spectrum every 'n' passes; this would also serve for synchronising purposes. There would be no need to accurately control the rotational speed as long as it was steady in the short term. The reference spectrum would calibrate the span and the end points; it could also calibrate the spectral amplitude response of the photo-detector.

A small gas-filled discharge tube, pulsed by an ignition transformer, would suffice for non-critical calibration.

That was how the early Hardy spectrophotometers did it back in the day when photomultiplier tubes were large rare expensive beasts surrounded by insanely high voltages and lots of precision megohm resistors.

I still have a few small mirror bits from taking one apart long ago.

Today with ultra cheap LCDs and some with piezo shift facility the trend is towards making a 2D spectrum on a standard rectangular CCD sensor with high dispersion on one axis from a grating and low dispersion in the other from a prism. Mapping the entire spectrum into a 2D pattern.

Echelle spectrograph is the keyword you want. Oxford instruments make rather high end ones but you don't need anything so fancy for this.

A neon lamp or a mercury vapour one will do for a wavelength reference. It's hard to get a sodium lamp small enough and at a good price.

Simplest solution is to limit the aperture that you let the light in through which is normally focussed onto a narrow slit anyway. You might get a bit of an issue with readout smearing but it probably won't be too bad.

Please bear in mind that my experience with spectroscopy the problem was mostly getting enough light to have *any* signal to noise.

In extremis the measure now button could just move a spring loaded mechanical shutter that normally blocks the light path.

Unless the thing is imaging a nuclear blast, steel furnace or an arc lamp then I don't think light intensity is likely to harm a modern CCD. There are hot mirror and anti-UV low pass filters to protect such equipment from hostile radiation.