BSDF eta values stored in .spd versus values reported in traverse

[johnson-c]

I'm missing something, I looked through the documentation and don't see any mention of this. The values of eta and k stored in the "..spd" values differ when I traverse the scene and look at "bsdf.eta.value" and "bsdf.k.value". I'm using the RBG render so I understand there is some averaging that takes place to convert the spectrally resolved values into RGB but I get different values from those stored in the .spd even if I make those values constant with wavelength. Where is this conversion done?

e.g
; Lambda (A) n k
;-----------------------------------------
309.95 3.99
...
...
885.57 3.99

In [22]: params_scene['center_stack_b.bsdf.eta.value']
Out[22]: [[4.80749, 3.78343, 3.62797]]

[merlinND]

Hello @johnson-c,

Without knowing which spectrum plugin you are using, my best bet is that this is the conversion step you are seeing:

mitsuba3/src/core/xml.cpp

Lines 1230 to 1237 in 284ecf7

	/* Pre-integrate against the CIE matching curves. In order to match
	the behavior of spectral modes, this function should instead
	pre-integrate against the product of the CIE curves and the CIE
	D65 curve in the case of reflectance values. */
	Color3f color = spectrum_list_to_srgb(
	wavelengths, values,
	/* bounded */ !(within_emitter || is_unbounded),
	/* d65 */ !(within_emitter && !is_unbounded));