Table 1. Table of symbols.

Symbol	Meaning	Units
As	Absorption rate in a small section of the photoreceptor	photons s−1
A7, A8	Absorption rates in R7 and R8	photons s−1
Dr	Rhabdomere diameter	µm
d	Degree of polarization
ΔS	Number of discriminable polarization angles
δ	Dichroic ratio of the rhabdomere
F	F-ratio of the facet lens
Fa(κ)	Absorptance for wide spectrum light, where κ = lrk(λmax) is the product of rhabdomere length and peak absorption coefficient
I(Q; θ)	Mutual information between opponent unit output and polarization angle	bits
k∥(λ)	Absorption coefficient for light of wavelength λ polarized parallel to the microvilli	µm−1
k⊥(λ)	Absorption coefficient for light of wavelength λ polarized perpendicular to the microvilli	µm−1
L(λ)	Spectral radiance	photons sr−1 m−2 nm−1
l	Total length of central rhabdomere pair (CRP)	µm
lr, l7, l8	Length of a rhabdomere / R7 rhabdomere / R8 rhabdomere	µm
λ	Wavelength of light	nm
M7, M8	Transduction rates	photons s−1
$M_7^{\left(\tau \right)},M_8^{\left(\tau \right)}$	Photons transduced per integration time τ by R7 and R8	photons
Ni	Photon flux incident at the distal extreme of R7	photons s−1
PS, PS7, PS8	Polarization sensitivity of a photoreceptor / of R7 / of R8
q, q7, q8	Contrast signal in a photoreceptor / R7 / R8
Q	Opponent signal
Ri(λ)	Spectral photon flux incident at the distal extreme of R7	photons nm−1
SNR	Signal-to-noise ratio
σin	Standard deviation of intrinsic noise
td	Photoreceptor dead time	ms
τ	Integration time	ms
θ	Polarization angle	degrees