Page 217 - Algae Anatomy, Biochemistry, and Biotechnology

200 Algae: Anatomy, Biochemistry, and Biotechnology

Irradiance (Flux Density)
2
1Wm 22 ¼ 10 24 Wcm 22 ¼ 6.83 10 lux (lx) at 555 nm
1lmm 22 ¼ 1lx ¼ 10 24 lm cm 22
1 photon at 400 nm ¼ 4.96 10 219 J
1Wm 22 at 400 nm ¼ 1Jm 22 sec 21 ¼ 3.3 mmol m 22 21 ¼ 3.3 mEinsten m 22 sec 21
s
1 photon at 500 nm ¼ 3.97 10 219 J
1Wm 22 at 500 nm ¼ 1Jm 22 sec 21 ¼ 4.2 mmol m 22 sec 21 ¼ 4.2 mEinsten m 22 sec 21
1 photon at 600 nm ¼ 3.31 10 219 J
1Wm 22 at 600 nm ¼ 1Jm 22 sec 21 ¼ 5.0 mmol m 22 sec 21 ¼ 5.0 mEinsten m 22 sec 21
1 photon at 700 nm ¼ 2.83 10 219 J
1Wm 22 at 700 nm ¼ 1Jm 22 sec 21 ¼ 5.9 mmol m 22 sec 21 ¼ 5.9 mEinsten m 22 sec 21

Radiance
22 21 2 22 21 22
1Wm sr ¼ 6.83 10 lm m sr at 555 nm ¼ 683 cd cm at 555 nm
Radiant Intensity
1Wsr 21 ¼ 12.566 W (isotropic) ¼ 4p W ¼ 683 cd at 555 nm

Luminous Intensity
1lmsr ¼ 1cd ¼ 4p lm (isotropic) ¼ 1.464 10 23 Wsr 21 at 555 nm

Luminance

1lmm 22 sr 21 ¼ 1cdm 22 ¼ 10 24 lm cm 22 sr 21 ¼ 10 24 cd cm 22

Geometries
Converting between geometry-based measurement units is difficult, and should only be attempted
when it is impossible to measure in the actual desired units. You must be aware of what each of the
measurement geometries implicitly assumes before you can convert. The example below shows the
conversion between lux and lumens.
If you measure 22.0 lux from a light bulb at a distance of 3.162 m, how much light, in lumens, is
the bulb producing? Assume that the lamp is an isotropic point source, with the exception that the
base blocks a 308 solid angle. Using Equation (5.15), the irradiance at 1.0 m is E 1.0 m ¼ (3.162/
2
1.0) 22.0 ¼ 220 lm m 22 . On the basis of steradian definition [Equation (5.4)], we know that
2
1 sr cuts off a spherical surface area of 1 m at a distance of 1 m from the source. Therefore,
22 21 21
220 lm m corresponds to 220 lm sr . The solid angle of the lamp is equal to 2p hr [Equation
(5.4)], where h is the height of the spherical calotte, and corresponds to 2p [1–cos (360 2 30/
2)] ¼ 12.35 sr, while the shadowed solid angle corresponds to 0.21 sr. Therefore, the total lumen
output will be 220 lm * sr 21 12.35 sr ¼ 2717 lm, (Figure 5.13).

PAR DETECTORS
Photosynthetic irradiance is the radiant flux density of PAR and is expressed as the radiant energy
(400–700 nm) incident on a unit of surface per unit time. A PAR detector is typically an irradiance
detector that is equally sensitive to light between 400 and 700 nm and insensitive to light outside
this region. Irradiance is now internationally expressed in moles of photons per unit area and
per unit time as mmol m 22 sec 21 (formerly mEinsten m 22 sec 21 ), where 1 mmol (mEinsten) corre-
sponds to 1 micromole of photons, that is, 6.02 10 17 photons, at a given wavelength. Modern
instruments measure in situ irradiance flux densities directly in mmol m 22 sec 21 .