Question

Question: I am working on a bid for an upcoming lighting project. Other than basic resendential lighting I know little about lighting and can use some assistance.This is a small freestanding neighborhood ice cream shop. The parking lot is adequatelly lit but the front of the building is dark and and at night its hard to tell they are open. My current plan is to install two metal halide fixtures on two existing 4" metal light posts in front of the building. I will be mounting these fixtures at 15' and the posts are 45' from the front of the building. I'm thinking 175 watts each will get the job done but I'm really just guessing.The photometric chart for the fixture I'm considering shows a factor of 1.0 at 175 watts at 15' and 40'. I'm not sure whats that means or if it will work for me.Any assistance from someone that does these all the time would be helpful. I would like to make this a learning experience since this is an area than I am real weak in. The lighting "expert" at my supply house was of no help.Thanks

Answer 1

Understanding the Photometric Chart

The photometric chart provides information about the light distribution and intensity of the fixture. The "factor of 1.0" at 175 watts at 15' and 40' means that at those distances, the fixture produces 100% of its rated light output.

Calculating Light Levels

To determine if 175 watts per fixture will adequately light the building, you need to calculate the light levels at the front entrance. The formula is:

Illuminance (foot-candles) = (Luminous Flux (lumens) / Distance (feet)^2)  Coefficient of Utilization

Luminous Flux

The total light output of the fixture, which for a 175-watt metal halide fixture is typically around 12,000 lumens.

Distance

The distance from the light source to the point of measurement (the front entrance), which in your case is 45 feet.

Coefficient of Utilization

A factor that accounts for the light lost due to absorption and reflection. For a small outdoor area with minimal obstructions, a coefficient of 0.5 is reasonable.

Example Calculation

Illuminance = (12,000 lumens / 45^2) 0.5 = 0.69 foot-candles

Adequate Lighting Levels

For commercial building entrances, a minimum illuminance of 2 foot-candles is recommended. Based on the calculation above, 175-watt metal halide fixtures will not provide sufficient lighting.

Recommended Solution

To achieve adequate lighting levels, consider using higher wattage fixtures. A 250-watt metal halide fixture with a luminous flux of around 18,000 lumens would produce an illuminance of:

Illuminance = (18,000 lumens / 45^2)  0.5 = 1.24 foot-candles

This is closer to the recommended minimum of 2 foot-candles and should provide adequate visibility for the building entrance.

Additional Considerations Light Distribution

Ensure the fixtures have a wide beam angle to distribute light evenly across the building façade.

Shielding

Use fixtures with shielding to prevent glare and light pollution.

Energy Efficiency

Consider using energy-efficient fixtures, such as LED or fluorescent, for long-term savings.

Maintenance: Plan for regular cleaning and maintenance to ensure optimal performance and longevity of the fixtures. Flag for review

Answer 2

Understanding the Photometric Chart

The photometric chart provides information about the light distribution and intensity of the fixture. The "factor of 1.0" at 175 watts at 15' and 40' means that at those distances, the fixture produces 100% of its rated light output.

Calculating Light Levels

To determine if 175 watts per fixture will adequately light the building, you need to calculate the light levels at the front entrance. The formula is:

Topic: Exterior Lighting