Continued Evolution of the Design

A project log for Compact, $25 spectrometer

AMS's new AS7265X 3-chip set promises a compact, 18-channel, 20 nm FWMH spectrometer for less than $25

Kris WinerKris Winer 07/08/2018 at 17:170 Comments


Now that everything works, I am in the process of refining the design to optimize utility. I got the latest iteration (v.02b) back from OSH Park and built one.

Here ready for testing.

A closeup view.

I added 2.54-mm diameter mounting holes at the bottom. I put all of the IO on one side to make it easier to solder a wire bundle or add a connector. I added a footprint for OSRAM's SFH4735 broad-band IR led. And I added what I thought was a switch to more easily go from I2C to UART mode but the SPVM110100 I selected is really a tactile button, so in the next revision (v.02c, I hope the last) I replaced it with my old standby the SSAJ110100 mounted on the back of the board. In fact, I had to remove the pesky SPVM "button" since I also got the footprint wrong and it was shorting the board! And, since I ran out of the MX25L4006E 4 Mbit SPI NOR flash I was using I switched to the W25Q80BLUX1G 8 Mbit SPI NOR flash with same 2 mm x 3 mm USON footprint, of which I have an abundance. I moved the (green) indicator led to just under the SPI flash; this is useful for indicating data ready interrupts when using the spectrometer. I kept the two 0630 footprints on the board, and kept them unpopulated, in case I (or users in general) want to add a specific light frequency (like UV or IR, for example) as an additional source.

The OSRAM SFH4735 is purported to be a broadband IR source:

with a convenient blue glow (peak at 440 nm) to aid illumination (since the broad band IR above 700 nm can't be seen by humans). This is a great idea, but the problem is the IR is only significant when the current is cranked up pretty high (350 mA in the output spectrum above). My little Dragonfly has an output maximum of 150 mA. But the AS7265X maximum source led current is only 100 mA anyway. Let's see what we get.

First of all, I procurred the wife's portable vanity mirror and made use of my $5 third hand to create a scientific laboratory for testing the latest version of the spectrometer:

The above is using both the broad-band Luxeon-Z-ES 5700 K 90 CRI led and the OSRAM SFH4735 at 50 mA output current.

Under these conditions (16x gain, 100 ms integration time, 50 mA sources, ~6 cm distance from source to mirror), this is what I see:

These spectra are averages over ten or twenty seconds to eliminate variations due to fluctuating source current and anything else. I really should solder the wires onto the spectrometer, since the j-hooks are a bit dodgy. At 50 mA, the broadband IR (IR50) seems to add a bit to the white light (BB50) source (~10x at 850 and 940nm) above 750 nm, as it should, as well as adding to the blue end of the spectrum. The OSRAM SFH4735 is fairly expensive, and it is not yet clear that it is adding high value; more testing will tell.

The IR spectrum looks a lot like the spectrum in the SFH4735 datasheet except that the peak at 440 nm is suppressed.  This might have to do with the low current.

Looks like the spectra just increase monotonically with increasing led current, so any differences between what I measure and the data sheet spectrum are due to variations from led-to-led, or different sensitivity of the spectrometer at different light frequencies. Well, this is the whole point of using a good reference, and I am assuming the vanity mirror is such a reference (at least it is better than white copy paper).

So let's measure something with the combination of white broadband and IR broadband sources at 50 mA. This is what aluminum foil gives:

This is the spectrum measured from aluminum foil divided by the source spectrum using both the broad-band sources (white light and IR) at 50 mA and the spectrometer settings above. Aluminum has a pretty high reflectance that is nearly constant over this spectral range so should give a horizontal line less than one (aluminum foil is not as reflective as a silvered mirror), and this is what we see, more or less. The line is at ~0.6 with most points lying within +/- 0.2 of 0.6

I have noticed that the relative strength of the signal at various frequencies (435 nm in particular) is somewhat sensitive to the specific orientation of the spectrometer to the test object. This might be due to the location of the interference filter for these frequencies inside the spectral elements in general, or due to the arrangment of the filters in this specific sensor on this board. But there does seem to be an acceptance angle problem of some sort. But rather than the three separate 6-channel sensors thwarting any attempt to use the AS7265X system as an integrated spectrometer as some feared, it looks more like one or two individual filters are hyper-sensitive to alignment issues, here the 435 and 535 nm channels.

Still, the spectrometer works well, is easy to use, and is very inexpensive. Only additional testing, and application to material discrimination/identification, will determine its ultimate utility.

For those following the design development, here is the latest iteration submitted to OSH Park. If anyone orders and assembles some of these, please let me know!