Sorry for not being clear.
Theoretically any grayscale image (one component) or image which is visually grayscale (R,G and B components but R=G=B). It'd be no specific format, but in my use case PNG. Bits per component reach from 8Bit over 16Bit and 24Bit to 32Bit.
So for example I have a 24Bit PNG, a 16Bit PNG and 32Bit PNG... With the above conditions they are grayscale.
Destination format is a 8, 16, 24 or 32Bit RAW format as previously described. This definitely has only one component (grayscale).
It obviously wouldn't make sense to convert 8Bit to 16Bit or anything like that because it would effectively still be 8Bit;)
If possible then a conversion from Destination (RAW) to Source (any format) would be really useful, too:)