STELLA: A DIY Handheld Spectrometer

STELLA (Science and Technology Education for Land / Life Assessment) is a do-it-yourself handheld spectrometer developed by Paul Mirel. An instrument for science, education, outreach, and engagement.

STELLA spin

Build Your Own STELLA

The instrument control panel
The measurement display screen
What’s inside
Where STELLA “looks” (field of view)
STELLA can ride on a drone
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What does STELLA measure?

1. Light intensity, in the visible wavelengths (in nanometers): 450, 500, 550, 570, 600, 650

The visible light intensity is measured in microwatts per centimeters squared, with error bars of +/- 12% of the value. The sensor selects the specific wavelength bands by using a set of silicon thin-film interference filters, to a precision of +/- 5 nanometers. The bands are centered around the wavelengths listed above, and the bandwidth of each band is +/- 20 nanometers full-width half-maximum around the band center, in a Gaussian distribution of senstivity. Silicon is a hard material with low thermal expansivity, so the sensor characteristics are stable over a broad temperature range, as well as over the life of the sensor. The sensor’s field of view is cone-shaped, with a cone angle of +/- 20\’ba for a total field of view of 40\’ba. The sensor is the as7262 visible spectral sensor built by ams-OSRAM, on a small circuit board built by Adafruit Industries.


2. Light intensity (in microwatts per centimeters squared), in the near infrared wavelengths (in nanometers): 610, 680, 730, 760, 810, 860

The near infrared light intensity is measured in microwatts per centimeters squared, with error bars of +/- 12% of the value. The sensor selects the specific wavelength bands by using a set of silicon thin-film interference filters, to a precision of +/- 5 nanometers. The bands are centered around the wavelengths listed above, and the bandwidth of each band is +/- 10 nanometers full-width half-maximum around the band center, in a Gaussian distribution of senstivity. Silicon is a hard material with low thermal expansivity, so the sensor characteristics are stable over a broad temperature range, as well as over the life of the sensor. The sensor’s field of view is cone-shaped, with a cone angle of +/- 20\’ba for a total field of view of 40\’ba. The sensor is the as7263 near infrared spectral sensor built by ams-OSRAM, on a small circuit board built by SparkFun Electronics.


3. Surface temperature, in degrees Centigrade. This sensor measures a far infrared light spectrum to produce a spectral curve. This curve is fit to a black-body thermal emission curve, to derive the surface temperature. This sensor is calibrated to objects that are emissive (not shiny metal surfaces). The temperature reading is good to +/- 0.5 \’baC. The sensor’s field of view is cone-shaped, with a cone angle of +/- 17.5\’ba, for a total field of view of 35\’ba. This sensor, chosen for a field of view that approximates that of the spectral sensors, is the MLX90614ESF-BAA built by Melexis. There are other versions of this sensor with larger fields of view.


4. Air temperature, in degrees Centigrade. This sensor measures the ambient air temperature to an accuracy of +/- 0.25 \’baC. It measures the semiconductor conduction-band quantum energy band-gap to derive the temperature. This method of measurement is accurate across a wide range of temperatures (-40 to +125 \’baC) and is stable over the life of the sensor. This sensor is an MCP9808, manufactured by Microchip Technologies Inc., set on a small circuit board built by Adafruit Industries.


5. Ambient conditions: Relative humidity, barometric pressure, altitude, and air temperature. This sensor measures those four parameters, though the air temperature measurement is less accurate than that of the MCP9808, so we do not record this sensor’s air temperature reading. The relative humidity measurement is good to +/- 3% and the barometric pressure reading, in hectoPascals, is good to +/-1 hPa. The altitide measurement is uncalibrated, so the absolute value is not accurate. The precision of the altitude measurement is better than 0.1%, so we include it to allow data marking by altitude excursion (a quick rise and fall) if the STELLA is in use on an arial drone. In this way, the drone data and the STELLA data can be synchronized. This is particularly helpful for gathering GPS readings from the drone to tag the spectral data from STELLA, for agricultural and ecological use. The sensor is a Bosch BME280, on a small circuit board by Adafruit Industries.


6. Time. We manually set the real time clock on the STELLA to Coordinated Universal Time (UTC) to avoid confusion of time zones and daylight savings time. After we set the clock, it will continue to keep time, powered by the backup battery, even when the STELLA is off. The accuracy of this clock chip is +/- 2 seconds per day, about +/- 12 minutes per year. The real time clock chip is a PCF8523, manufactured by NXP Semiconductors, on the Adalogger data logger module built by Adafruit Industries. }

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