Details of Irradiance List tag for Sunshine sensor in exif data of Sequoia


#14

@Jman841, thanks. This makes sense.

@domenzain, its great news that Parrot is finally releasing some documents.


#15

@domenzain, lets say I get correct CH0 and CH1 counts and I also make up for the gain and integration time factor. Now what information does this count convey to me?

  1. How can I relate this count with irradiance having units of W/m^2/sr?

  2. How can that irradiance amount be used for the radiometric calibration process?

Overall I am a bit confused about how the sunshine sensor count works with radiometric calibration process. It would be great if you can refer some paper or other informative material.


#16

I had assumed, probably very naively that it would be as ‘simple’ as working out an EV (Exposure Value) for the irradiance sensor using it’s gain, exposure time and aperture. And then working out the EV for the captured image and using the ratio of EVs to normalize captured image values to the irradiance sensor and then dividing the values to get a reflection percentage for each pixel in the captured image.


#17

@seanmcleod, I don’t think it’ll be that simple. From tests I’ve done, there’s no simple linear relationship between ch0 values (even if corrected for gain and int_time) and the pixel values in photos taken under different light conditions. So ratios won’t work.
Also, there is no simple linear relationship between your image exposure (ISO or shutter speed) and it’s pixel value either (Shutter speed and ISO correction)… So, we need Parrot to give more info in order to be able to apply any corrections… hopefully, they will.


#18

@seanmcleod, Js_yellow is correct. There doesn’t seem to be a simple linear relationship even for exposure correction. Multiplying by the ratio of EV values doesn’t work.


#19
  1. You cannot without further measurements. Nor would you need to do so in general: a reflectance calibration target normalizes the reflectance directly to a known standard.
    You can however relate the count to arbitrary units proportional to W sr^-1 m^-2 (note the proportionality symbol in my equation above).
  2. What do you mean precisely by “radiometric calibration process”?

Have a look at Laboratory Measurement of Bidirectional Reflectance of Radiometric Tarps, it will clear things up. There are many other reference materials for this.


#20

This would work if both sensors had identical solid angles and sensitivities for perfectly Lambertian surfaces at nadir.


#21

As I mentioned above, there are some documents on the way. I am unable to provide an exact release date at present.


#22

We’ll have to keep waiting… Been told about the “incoming” documentation for months…


#23

May not always work, but in this case, there seems to be a fairly straightforward relationship between ch0 counts and pixel values.


#24

@pk123, your experiment results are interesting. However, I have a few questions.

  1. I believe you had to acquire images which had sunshine sensor value (CH0 count) stretched across the range. How did you do that?

  2. You have taken only 1 reflectance panel and mean of digital number for that panel is plotted againt the CH0 of sensor?


#25

@muzammil360

  1. I took photos over 5 hours outside. Sequoia on a tripod facing straight down, sunshine sensor facing straight up (i.e. not normal flight conditions)

  2. yes, 1 reflectance panel (75% reflectance), each data point is the average pixel value of the entire reflectance panel.

I should mention that I kept ISO and shutter speed constant throughout.


#26

@pk123, did you do any black level compensation or vignette compensation?


#27

@muzammil360,
no, these are the raw pixel values, no adjustment.
Panel was centered and at same location in all shots, so I wouldn’t expect vignetting to affect the trends significantly


#28

@pk123 given the 5hr interval the sun’s elevation would’ve changed. Did you not apply Lambert’s Cosine Law to the CH0 counts to take into account the varying angle?


#29

@seanmcleod, no. Since the sunshine sensor is not facing the sun (it point straight up and it was winter so the sun was never straight above), I wouldn’t expect the Lambert’s Law correction to work. It is possible that because the sunshine sensor receives the sunlight at an angle (same as the ground), it already accounts for the varying sun position during the day. I keep meaning to try this again and see it I get the same relationships.


#30

@pk123 but even if the sun’s elevation was never at 90 degrees to the sunshine sensor over a 5hr period the angle would’ve changed which would change the amount of irradiance measured by the sunshine sensor.

For example assume there was a 10 degree elevation change for the sun from 40 to 50 degrees over the 5hr period, that’s the difference between cos(40) = 0.77 versus cos(50) = 0.64. So I would expect to see a difference in the CH0 counts over the interval.

Now the Sequoia sensor has a GPS and IMUs (one for the sunshine sensor and one for the RGB&Multispectral sensor) so in theory they could work out the sun’s angle relative to the attitude of the sunshine sensor in realtime but I’m pretty sure they don’t. Instead they simply record the attitude info in the IrradianceList tag so that you can use it in post processing.

For example in Pix4d you’ll notice the following info comments during post processing:

Info: cos(sun, irradiance sensor) 0.573237 direct sunlight fraction 0.933341 correction 0.853415


Questions about Radiometric Calibration and data correction
#31

@seanmcleod, i wasn’t saying that the sun angle wasn’t changing nor that the ch0 count didn’t change during the 5h interval. I’m only suggesting that since the sunshine sensor is receiving the sun light at the same angle as the ground is receiving it, it may not be necessary to do an angle adjustment. The linear relationship I observed seem to support that idea.


#32

@pk123 thinking about it a bit more I agree that in your setup both sensors (irradiance sensor and multispectral sensors) are seeing the same scale factor based on Lambert’s Cosine Law.

I had originally only focused on this part of your earlier comments: “Since the sunshine sensor is not facing the sun (it point straight up and it was winter so the sun was never straight above), I wouldn’t expect the Lambert’s Law correction to work.”


#33

The camera itself doesn’t do much in terms of computing and data processing.
When it comes to sun angle calibration, to the best of my knowledge, only Pix4D knows how to do it and actually does it.
see Radiometric Processing and Calibration

Allows the users to calibrate and correct the image reflectance, taking the illumination and sensor influence into consideration. It is possible to choose the type of radiometric correction to be done:

No correction: no radiometric correction will be done
Camera only: corrections will be applied for the parameters that are written in the EXIF metadata and relate to the camera (vignetting, dark current, ISO, etc…).
Camera and Sun Irradiance: corrections will be applied for the camera parameters from the point above as well as for the sun irradiance information written in the XMP.Camera.Irradiance EXIF tag.
Camera, Sun Irradiance and Sun angle: corrections will be applied to take into account the sun position, as well as the camera information and the irradiance data. This option should only be chosen for flights that were done in clear sky conditions.

I don’t know exactly what youexpect from your measurements in terms of accuracy and precision but the sun angle correction seems to be the ultimate step when you want to have dead-on measurements.
We recommend the camera and the sunshine sensors to be rigidly mounted on the drone in order to maximize the accuracy of the measurements. It is best when the two modules have a constant angles between them.
I hope this helps