If you imagine you had a perfect lamp that produced an equal amount of light in all directions, the spread of light would be spherical. If you cut a section through the middle of the sphere of light, you would see a circle with the light source at its center. However, this is theoretical, and real lamps and luminaires do not produce a perfectly even spread of light—in fact, most luminaires are designed to produce something other than a spherical spread of light. Graphical representations can be used to show the pattern of light a luminaire produces.
A fluorescent tube is as close as you may normally get to a lamp that produces light equally in all directions. The proviso is that because it is a linear lamp the spread of light is cylindrical rather than spherical. When viewed end-on, a fluorescent tube by itself produces an even distribution of light over a full 360 degrees. However, a fluorescent lamp is normally used as part of a strip luminaire, and this affects the spread of light from the lamp. Obviously, no light can pass through the housing, which causes a shadow, but some of the blocked light will be reflected back, resulting in additional light in certain directions—the spherical distribution of light has been altered significantly. This information can be described with a polar intensity diagram, which represents a section slice through the luminaire, with the spread of light shown as a curved line.
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Top left
The polar intensity diagram is produced by taking lighting measurements all the way around the luminaire and drawing a curve —the red lines in these diagrams describe the light intensity on a plane through the center of the luminaire. The influence of the luminaire on the shape of the blue intensity curve is clear to see. The top of the curve is flattened because the housing is interrupting the light going upward from the lamp, and some of that light is reflected downward, giving a slight bulge at around 30º from the vertical. This illustration and the two that follow include drawings of luminaires to make understanding the diagram easier. However, in practice the luminaire is not usually shown in a polar intensity diagram.
Center left
In this example the red line represents the polar intensity curve for the fluorescent strip after it has been fitted with a polished reflector. The reflector stops all light from escaping upward and redirects it downward, creating a real direction to the distribution of light. This diagram can also help you to visualize other features of the luminaire. You will see that the intensity curve does not extend above 60º from the vertical. This is because the reflector is preventing any light escaping at a higher angle. If you positioned the luminaire near a vertical surface, no direct light would hit the surface above this 60º line. This could result in a very visible shadow line.
Bottom left
In the previous two examples the spread of light has been symmetrical—the same on either side of the vertical. Here, the reflector shape has been designed to produce an asymmetrical distribution of light. This polar intensity curve shows a clear peak on the right-hand side. You will also see that the luminaire produces light up to about 75º from the vertical, which means there would be a much smaller area of shadow if it were placed near a vertical surface. This kind of asymmetrical reflector is often used to provide an even illumination for vertical surfaces. 105º 90º 75º 60º 60º 75º 90º 105º 45º 30º 15º 0º 15º 30º 45º 105º 90º 75º 60º 60º 75º 90º 105º 45º 30º 15º 0º 15º 30º 45º 105º 90º 75º 60º 60º 75º 90º 105º 45º 30º 15º 0º 15º 30º 45º
Top right
This illuminance cone diagram for a 50 W low-voltage spotlight records a minimal amount of data. The lamp or luminaire is assumed to be positioned at the top center of the chart, facing directly downward (the direction known as the nadir). All measurements are taken on a horizontal plane perpendicular to the lamp. The numbers on the left show the distance from the lamp in feet at which the measurements were taken. The right-hand figures show the foot- candle level at those distances. For a spotlight, these numbers will be the peak, or maximum, foot-candle level. With this chart there is no way of knowing how even the foot-candle levels are across the beam, or whether the peak is in the center of the beam. The beam angle of the lamp is described by the manufacturer as being 40º. However, that may be an approximation, so the center column of numbers shows the actual beam diameter at different distances. It is important to realize that these measurements do not refer to the total spread of the light; the light does not stop beyond the 40º line. The beam angle quoted for the lamp actually relates to the angle at which the light level will be 50% of the peak —this is referred to as the half peak beam angle.
Bottom right
The half peak beam angle used to describe a spotlight is best understood with a polar diagram. In this example, increasing distance from the chart origin (the junction of the 0º and 90º lines) relates to increasing intensity. For this luminaire the peak output is not perpendicular to the spotlight, it is slightly to the side at around 20º from the vertical. For this luminaire, we would say the half peak beam angle was 102º—the angle at which the output is half the maximum output. In the case of this luminaire the light falls off very quickly above the half peak angle. Other luminaires may have very different characteristics.