I am trying to convert pixel DNs to reflectance from a Planet BGRN Analytic Scene (from 2016, I can't tell what level it is processed to). However, I can't find info on the exo-atmospheric irradiance of the 4 bands. Is this available somewhere?
If the file contains "DN" in the name then it is not scaled radiance and there is are no coefficients available to scale to scale to exo-atmospheric irradiance.
If the product is the "analytic" asset from the PSScene4Band type the filename should include "_3B_AnalyticMS" and it is scaled radiance.
If the product is the "analytic" asset from the PSOrthoTile type the filename should include "_BGRN_Analytic" and it is scaled radiance.
If you have a scaled radiance product, you can multiple by 0.01 to convert the 16bit pixels values to watts per steradian per square meter (W/m²srμm).
If you want to convert to convert to top of atmosphere reflectance, you will need to multiply by the value of the "reflectanceCoefficient" from the associated _metadata.xml file. Note that you do not compose the radiometricScaleFactor (0.01) and the reflectanceCoefficient - just multiple the original pixels values from the analytic files by hte reflectanceCoefficient. This ought to be covered page 32 of https://www.planet.com/products/satellite-imagery/files/1611.09_Spec_Sheet_Combined_Imagery_Product_Letter_DraftV3.pdf or perhaps there is a more recent version that did not come up in my google search.