TSL2591 - making sense of readings

Other Hardware Sensors
1

Hello, all. I am using the TSL2591 and get readings for broadband, lux, and IR, however, I don't understand the IR readings because there is no unit and consequentially I can't make sense of it. Is there a formula to convert it to a measured value with a unit? I am using the following config:

void configureLightSensor() {
tsl.setGain(TSL2591_GAIN_MED);
tsl.setTiming(TSL2591_INTEGRATIONTIME_100MS);

Serial.println(F("TSL2591 configured:"));
Serial.print(F("Gain : Medium\n"));
Serial.print(F("Integration Time : 100ms\n"));
}

example readings of mine:
Full Spectrum : 630 (broadband)
Infrared : 356
Lux : 277 lx
Thanks.

2

The sensor is not calibrated. However, the sensor data sheet gives typical sensitivities toward different light sources and intensities, with light intensities measured in units of microWatts/cm^2.

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For meaningful readings with your light sources, calibrate the sensor against some standard reference or a certified professional instrument.

3

Thanks for the speedy reply, I am not needing my sensor to be super calibrated because it will just need to be in the ballpark of the proper reading, but I don't understand the datasheet or the image you uploaded as I am very new to using sensors especially light sensors. Could you help shed light on the image and how to make the equation from it? Thanks.

4

Table 8, row "Irradiance Responsivity" specifies a conversion factor in units of counts/uW/cm^2 for white light as defined in the Table 8 footnote (a white LED with color temperature 4000 K).

For that type of white light, divide counts by the conversion factor to get uW/cm^2 intensity.

Whether those physical units of intensity are useful depends on what you intend to do with the sensor.

For an excellent general overview and tutorial on sensor calibration, see

5

I plan to use my sensor in daylight so it will be detecting IR from the sun. Is 850nm in table 8, uv light? If so does that mean my equation is rawcounts/257.5 = μW/cm2 or rawcounts/154.1 = μW/cm2? Sorry if this seems to be a dumb question, I am really unexperienced with light and sensors.

6

It would be a good idea to do some reading about the spectrum of sunlight, what the units mean, etc.

850 nm is a light wavelength in the Near IR region (NIR) and invisible to the human eye.

In the TSL2591 data sheet, Figure 8 defines for calibration purposes "850 nm" as the light from an IR LED, which peaks at 850 nm and has bandwidth 42 nm (that is, emits light in wavelength range about 830 to 870 nm, peaking at 850 nm).

The channel 1 photodiode of the TSL2591 (CH1) detects light over the range of 500 nm to 1100 nm, peaking between 800 and 850 nm, as shown in this graph:

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The sun emits light over an extremely broad spectrum, from about 250 nm to 2500 nm, as shown in this graph, but the TSL2591 CH1 photodiode can detect only a narrow portion of it, with quite a bit in the visible region (500 to 700 nm).

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So, the question remains: what do you want to do with the numbers produced by the TSL2591?

7

I would like to put a unit on my IR readings so that instead of a number they become a measure people will understand.

8

You have to understand what the readings mean. Then,you can figure out how to publish something that other people understand.

Why are you interested in IR?

9

I am doing a project for observing an environment and IR came with the sensor so I thought I might as well use it. I am getting raw count values for IR but I want to convert it into W/cm² for instance, I don't know this area very well and was hoping if you could give me the equation in the form of input: raw, output: some amount of W/cm²? Is that possible? Thanks.

10

What do you not understand about this statement, from post #4?

In C/C++ that would be

intensity = counts/conversion_factor;

If these concepts are too much trouble to absorb right now, consider starting with simpler projects and work your way up.

11

No, not too much trouble, all I have to find is conversion factor:
image
Now, assuming this is the referred table and 850nm is IR for CH0 the conversion factor is 257.5 and for CH1 it is 154.1? Also what does CH0 and CH1 refer to? Thanks.

12

Defined in data sheet, and post #6.

14

OK, so correct me if I am wrong, it seems CH1 is best for Near IR(700nm-1300nm). So my equation now becomes:
raw_counts/154.1 = amount of W/cm² ? Thank you.

15

microwatts/cm^2

and only if the light source has the same wavelength characteristics as the 850 nm LED described in the footnote to Figure/Table 8.

For any other light source, the sensor has to be calibrated by comparison to a certified professional lightmeter.