Radiant flux budget at the target. For any wavelength λ, the incident flux Φ_I(λ) splits into three parts:

Φ_I(λ) = Φ_R(λ) + Φ_T(λ) + Φ_A(λ)

• Φ_R — reflected energy.
• Φ_T — transmitted energy.

• Φ_A — absorbed energy.

• Reflectance (ρ, rho): ρ(λ) = Φ_R(λ) / Φ_I(λ) — the fraction reflected at each wavelength. Plotted vs. λ, this gives a spectral signature (also called spectral reflectance curve) for a surface — the foundation of all optical classification.