Introduction

This short report documents some simple light readings taken in my backyard. The purpose was to get some idea of the variation in irradiance throughout the course of a day. The trends in irradiance are more important that the readings themselves.

Methods

Readings were taken using a quantum meter from Apogee Instruments (www.apogee-inst.com) and the meter in question is Model QMSS-ELEC. This meter has been calibrated for electric lights but will still give useful relative measurements under sunlight, although it may be around 10% low. It should be noted that the sensor response is not perfect and shorter wavelengths are underestimated and longer wavelengths are overestimated, but as the spectrum of sunlight should stay fairly constant, these inaccuracies should not create any problems.

Readings were taken with the sensor in both a horizontal position and with the sensor facing the Sun.

Results

The measured irradiance levels are shown in Table 1:

Table 1: Irradiance levels during the day
Time	Irradiance (uE.m-2.s-1) Horizontal	Irradiance (uE.m-2.s-1) Facing the Sun
8:00 AM	900	1600
9:01 AM	1200	1850
10:00 AM	1450	1850
10:58 AM	1640	1855
11:35 AM	1660	1840
12:00 PM	1670	1850
12:50 PM	1630	1820
01:35 PM	1500	1820
04:00 PM	630	1430

Table 2 shows the solar altitude for the day of the measurements (October 6, 2002) and the location (Glenbrook, NSW, Australia).

Table 2: Solar altitude during the day (Data from Geoscience Australia: National Mapping Division).
Time	Altitude (degrees)
08:00 AM	30.38
09:01 AM	42.16
10:00 AM	52.15
10:58 AM	59.15
11:35 AM	61.12
12:00 PM	61.05
12:50 PM	57.62
01:35 PM	51.69
04:00 PM	24.73

Figure 1 shows both the irradiance levels and solar altitude plotted on the same graph.

Figure 1: Irradiance levels and solar elevation through the course of a single day.

Discussion

As discussed by Trevor-Jones (2002), solar altitude (or elevation) has two main influences on irradiance at the Earth's or ocean's surface: a) the lower the Sun is in the sky, the more atmosphere its light must pass through (Figure 2); and b) the lower the Sun is in the sky, the greater the area over which a fixed arc of light will be spread (Figure 3).

Figure 2: The amount of atmosphere through which sunlight must travel to reach the Earth's surface is dependent on the solar elevation. (Trevor-Jones, 2002).

Figure 3: The area over which a fixed arc of sunlight is spread is dependent on the solar elevation. (Trevor-Jones, 2002).

The readings taken with the sensor in horizontal position will be affected by both influences. Not only will there be greater attenuation when the Sun is lower in the sky, due to the light passing through more atmosphere, but what light reaches the sensor will be spread over a greater area when the Sun is lower.

When the sensor was facing the Sun, the light would not be spread at all and the sensor will receive the maximum available light. The maximum available light will be attenuated by passing through the atmosphere. When the Sun is lower in the sky, the light will have to pass through more atmosphere than it would when it is more overhead.

This simple set of readings show well the effect of solar elevation.

References

Trevor-Jones, A.G. 2002. Underwater Lighting Conditions. Reefkeeping Magazine 1(8) (www.reefkeeping.com)

Last updated: October 6, 2002