To determine the optimum tank depth for the assessment of spectral reflectance from suspended inorganic matter (SIM), a 1·10 m × 0.90 m × 0·67 m black-coated tank was used. A plate with the same coating was constructed to simulate various depths. Different concentrations of bentonite and red silt suspensions were produced and radiometric measurements were performed in the 400-900 nm spectral range. The results showed close relationships between optimum depth and suspension characteristics, e.g., at 5mg/l of bentonite the optimum depth was around 0·63 m and at 140 mg/l it was 0·41 m. For red sill, at 140 mg/1 the optimum tank depth was greater than 0·41 m too. Before each experiment the optimum tank depth must be assessed to produce spectral reflectance of SIM.