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**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.

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