5. Why do we prefer units of "irradiance" to "intensity" when measuring photosynthesis?


Light intensity Vs irradiance

In common usage, light "intensity" is used to refer to the brightness of a light source. When measuring photosynthesis or selecting light bulbs at home, light intensity is not always the most useful measure.

The difference can be 'illuminated' by considering the common unit, the watt, used to measure brightness of electric light bulbs.. A "watt" is a measure of the energy output of the bulb. This makes selecting the correct light bulb for a particular setting somewhat subjective. For example, when designing a new room, we must consider the size of the space to be illuminated and the height of the ceiling when selecting the number and brightness of the light bulbs. In a small space, a 60 watt bulb may be adequate, whereas in a larger space a 150 Watt bulb may be necessary. Often, we resort to trial-and-error to determine which bulb will be suitable.

Ultimately, we base our decision upon the level of illumination obtained, i.e., the amount of light actually striking the surface of objects in the room. This is called the "irradiance level, " which you can see depends upon the intensity and distance of the light source.

Obviously, the rate of photosynthesis will depend upon irradiance, the amount of light striking the leaf, rather than light intensity, the brightness of the bulb. A 60 Watt light placed very close to a leaf would provide more light for photosynthesis than a 100 watt bulb located much further away.

Units of irradiance

The most common units of light irradiance are "uEinstein /m2 /min." An "Einstein" is equivalent to 1 mole of photons, the fundamental unit of light. You will recall from Intro chemistry that a "mole" is equivalent to 6.02 x1023 particles of a substance, such as atoms, molecules or photons. In this case the units are "microEinstein" (uEinstein; millionth of a mole of photons)--still a very large number of photons (1 uEinstein = 6.02 x 1017 photons).

Note that irradiance also encompasses units of area (square meter, m2) and time (min), thus giving the amount of light energy striking a 2-dimensional surface over a period of time.

(For biology majors) 5a. Why is the design of the photosynthesis apparatus not ideal for comparing irradiance levels of two different types of leaves?

A. The leaf chamber is not large enough to hold an entire leaf.

B. Leaves of different size can be placed within the chamber.

C. The light sensor does not measure all wavelengths of light.

D. The light sensor actually measures the amount of light passing through the leaf.

Click here for an explanation of this question.


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