Lighting Unit Comparison Scope

The objective is to determine the merits of using 150W HID lighting units, in various combinations, against 600W HID lighting units and other light sources.


The basis for most calculations will be the inverse square law:
The intensity of light radiating from a point source is inversely proportional to the square of the distance from the source.
So an object (of the same size) twice as far away, receives only one-quarter the energy (in the same time period).

lighting intensity and distance

lighting intensity and distance

If we take the intensity at 1d as 1
At 2d the intensity = 1/22 = 1/4
At 3d the intensity = 1/32 = 1/9

This means that if we double the distance from the source the intensity reduces to one quarter, if we triple the distance the illumination reduces to one ninth etc.
Using this law we can also derive the following:
• Reducing the distance by 0.7 (√1/2) will double the intensity.
• Increasing the distance by 1.4 (√2) will halve the intensity.

If the source is not a point source (as in a lamp) the following is true:
If the size of the light source is

 HID Lighting Systems

Investigation Points

1. How many 150W lamps would it require to provide the same lighting levels as 1 x 600W lamp?
2. How many 150W lamps would it require to provide the same area coverage as 1 x 600W lamp?
3. How using a 150W system compares cost wise to achieve the same levels of lighting/coverage as a 600W system?
4. Does the lower running temperature of the 150W system mean that the growing level can be much closer than with a 600W lamp and therefore produce improved results?

 Lumen Levels

Based on the manufacturer’s details (see note 2) the output from a 150W lamp (17500 lm) is only 19.44% of the output from a 600W HID lamp (90000 lm). As the inverse square law proves, the intensity drops at the same ratio for both systems; this means that at the same distance from source the level from the 150W lamp will only be 19.44% the level from the 600W lamp.

figure 1

figure 1

To obtain the same reading as a 600W lamp located 5 Ft from source
a 150W Lamp would have to be located 2.205 Ft from source.

figure 2

figure 2

It also means that the area covered by the lamps to achieve the same readings would reduce from 25 Ft2 for the 600W lamp to 4.86 Ft2 for the 150W lamp.


1. For illustrative purposes we have used the manufacturer’s details for the Lumen levels as
being taken at 1 Ft from source. The ratio between the systems would remain the same at
whatever distance from source the readings were taken.
2. The Lumen levels used are for the lamps: NAV-T 150 super 4Y and NAV-T 600 Super 4Y.
Details are taken from the Osram website (

 Investigation Conclusions

1. As shown a150W lamp only gives 19.44% the output of a 600W lamp at the same distance from source (Figure 1). Therefore it would require 5 x 150W lamps to provide approximately the same levels as 1 x 600W lamp at any given distance from source.

2. By moving the 150W lamps closer to the growing level the area covered is reduced to less than 1/5th (Figure 2). Therefore it would require 5 x 150W lamps to provide approximately the same area coverage as 1 x 600W lamp at any given Lumen level reading.

3. Whichever way is used to achieve the same levels/coverage as a 600W lamp the number of 150W lamps required is 5. Therefore more power would be used running the 150W system and result in not only higher initial outlay costs to purchase the equipment but also in higher running costs.

4. The heat output of the 150W lamp is proportional to the output of the 600W lamp which means it can be mounted nearer to growing level without the heat having an adverse effect. However, as the inverse square law shows with regard to distance and light intensity, the 150W system has to be mounted closer to growing level anyway purely to achieve the same light levels as those of the 600W lamp. Therefore mounting the lamp closer to growing level is a necessity rather than an advantage.