Which formula estimates lighting needs for a plant surface?

• A. N = (light level X surface area)/effective flux
• B. N = (surface area)/(light level X effective flux)
• C. N = (effective flux)/(light level X surface area)
• D. N = (light level + surface area)/effective flux

Answer: (A)

Estimating lighting needs is about matching the total light required for a plant surface with what a fixture can deliver. The formula does this by first calculating the total light required: multiply the target light level by the surface area. That gives the total lumens the surface needs. Then divide by the effective flux of a single fixture (lumens it provides) to estimate how many fixtures are needed.

Using consistent units is key: light level in lux (or foot-candles), area in square meters (or square feet), and fixture flux in lumens. The result, the number of fixtures, makes sense because total light required scales with area and brightness, while each fixture contributes a fixed amount of light.

For example, if you want 200 lux on 4 square meters, you need 800 lumens total. If each fixture outputs 1200 lumens, you’d need about 0.67 fixtures, meaning one fixture to meet the requirement.

Other expressions would mix or misplace the factors, leading to incorrect units or proportions. The correct approach is to multiply light level by surface area to get total lumens, then divide by the fixture’s lumens to find how many fixtures are needed.