Light sensitivity notes

Eyes adapt to a huge range of illuminance, from the light cast by the full moon (around 0.1 lux) to full sunlight (100,000 lux or more). Some estimates for various light levels seen without supporting data on the internet suggest that "twilight" is around 1 to 10 lux. Full daylight is around 1000 lux with an dark overcast day perhaps down 100 lux.

I was not able to find much in the literature about bird behavior specifically related to daylight. One paper focusing on woodland species during breeding season measured bird activity by listening for their calls and found substantial activity approximately 30 minutes before local sunrise, typically peaking in the hour after sunrise, and diminishing over the next four hours. It may be reasonable to assume that species of concern in crop protection would exhibit similar behavior.

Anecdotally, when testing the light sensor one morning (April 4, 2022; in a wooded residential area of Kingston RI), I perceived birdsong beginning at 5:40 AM (0.1 lux), thought I might have seen a bird at 6:05 AM (10 lux), definitely saw birds moving in trees but not flying at about 6:15 (50 lux), and saw birds flying but not at our feeder at 6:20 AM (100 lux). The first bird stopped by the feeder at 6:24 (178 lux) and the sun peeked over the top of the trees at 6:40 (500 lux). NOAA's Solar Calculator says that "apparent sunrise" was 6:24 AM that day, so I guess that first feeding bird checked it's watch!

The scarecrow is programmed to "go to sleep" when ambient light levels are low and "wake up" when light levels are higher, leaving a range in-between that will not cause a transition, similar to how if you set your furnace thermostat to 65°F, it might not turn on until it reads 63°F nor turn off until 67°F were reached.

In the 2022 URI laser scarecrow, the sensitivity to light can be adjusted as one of the secondary settings. To help with repeatability, only 11 possible settings are available with the middle value being almost exactly the default. These settings are shown along the "x" axis of the following graph and the 0-4095 value read by the microcontroller is the "y" axis. A reading of 0 indicates light levels are probably below 1 lux. A full reading of 4095 indicates light levels of at least 200 lux, usually at least 400 (there is some variability between individual light sensors). To better cover the wide range of possible readings, the available sensitivity levels (x axis of the graph) are scaled exponentially.

You may notice that the light sensor (Excelitas Technologies IR-Bloc™ Ambient Light Sensor VTT9812FH silicon phototransistor) is moderately directional. Per its data sheet, its response will be down 50% when 50° off of center. So it will make a difference if the light sensor is oriented toward sunrise or away from it.

On a sample of 10 of the light sensors, I adjusted a diffuse 4000K multispectral LED light source so that the light sensor read as close to the default "sleep" level of 500 as I could get. Then I measured the same light source at the same distance with a meter (light source and either sensor/meter at opposite ends of a piece of PVC pipe). The values read off the meter were 8, 9, 10, 12, 12, 12, 15, 14, 16, and 25 lux which average to around 13 lux but note that the standard deviation is almost 5 lux.