# Reflectance Estimation

#81

@muzammil360 thanks for the reply nonetheless.

If there is anyone else that has experience of processing Multispec 4C imagery, can they weigh in with their thoughts as it would be much appreciated?

#82

Can someone explain a bit clearer how to get irradiance without the `SensorModel` tag? How do I apply the equation `count = A’ exposure gain irradiance + B’`? I can get irradiance from the sunshine sensor quite easily, but havent figured out how to do it for the Sequioa without the `A`, `B` and `C` coefficients from `SensorModel` tag.

#83

Simply solve for irradiance in your equation. `B` and `B'` are identical when there is no light and the remaining `A'` gets absorbed into `K`.

Chances are you really do not mean to do this, as a two coefficient model will necessarily be a worse fit.

Think of `C irradiance + B` as a more precise `B'` that explicitly says that it depends on `irradiance` .

#84

Hi @jeromeoc
Unfortunately without the SensorModel tag, it is impossible to get irradiance information. However, if you update your Sequoia to the latest firmware, you should get access to this tag.

Best regards

#85

so instead of using `I = f2(p-B/AeY + C)`, where `A,` `B` and `C` are the `SensorModel` tag values, I solve for
`I = f2(p/eY)` where:
`I`: Irradiance at sensor
`f2`: f-number squared (k)
`p`: raw pixel value (value)
`e`: exposure time (t)
`Y`: ISO value (K)

#86

Yes, the camera was updated half way through a survey season so the `A`, `B` and `C` coefficients for each band are the same through the lifetime of the camera? That way I can retrospectively go back and correct the images that were acquired before the firmware update?

#87

You’re right, the A,B and C are calibrated in factory and so remain constant for the lifetime of the camera.

#88

Perfect! Thanks for all your help

#89

To get Sequoia irradiance which is sunshine corrected (i.e. taking into account the values from the sunshine sensor) should I use `I' = Iseq / Isun` to scale it to the values of the sunshine sensor or just use the values that come from `I = f2(p-B/AeY + C)`?