Downward-looking photometer

The LI-COR model LI-210SZ photometer is used by the UVMRP as a simple indicator of surface reflectance. The photometer is pointed downward instead of skyward, and therefore responds to illumination reflected from the seasonal ground cover just below and around the Network's instrument array. The importance of this measurement is in determining the presence of snow cover.

Because of the photometer's unconventional orientation within the Network, UVMRP has chosen to apply a single, constant calibration conversion (30.69 mA per 100K lux) to all of these sensors rather than the specific constants for each sensor as furnished by the manufacturer. Since the stated calibration constants for the Network's photometers range from 27.86 - 33.51 mA per 100K lux, this introduces an additional uncertainty in the reported measurements of approximately 7%. This uncertainty, however, is more than acceptable for the instrument's intended use. To further differentiate the Network's photometric results from other LI-COR photometer results, UVMRP reports the sensor's output in units of Watts/meter2 rather than lux as is suggested by the manufacturer. This unit conversion is made using the 1 Watt = 683 lumens as recommended by the manufacturer. UVMRP does not maintain calibration of these instruments, as they are intended to provide only a relative indication of the presence of snow on the ground.

LI-COR response chart The LI-210SZ sensor uses a filtered (blue enhanced) silicon photodiode to provide a spectral response that matches the CIE photopic curve (having a spectral responsivity equal to the average human eye) within 5% with most light sources. This photodiode and filter combination is placed under a cosine-corrected acrylic diffuser to provide the proper response to radiation at various angles of incidence. This sensor does not have any internal heating system, and thus operates at ambient temperature (operating range: -20C to 65C; 0 to 100% RH). [Specifications and graph are from the LI-COR instruction manual.]

However, at the request of the researchers using this data, two sites, TX21 at Seguin and TX41 at Houston, have their photometers pointing skyward, as their potential for any significant snowfall is near zero. The TX21 photometer has a collimating tube attached, to prevent direct Sun from impinging on the diffuser.

The downward-looking photometer is connected to the vis-MFRSR datalogger, so its output is measured every 15 seconds and integrated into 3-minute averages, and its data is retrieved nightly in conjunction with the polling of the vis-MFRSR datalogger.