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Television Lighting: “Retrofit Ruminations”

by Bill Klages

Justifying Retrofit

Every broadcaster has moments when their interest is provoked by the thought of the merits of changing to solid state lighting in their facility. One of the primary attractions is the idea that the efficiency of the new LED instruments will result in significant saving in electrical energy. Estimating the Payback Period (of investment) is a quick and practical method to initially evaluate the financial benefit of a retrofit.  The procedure is very straightforward and easy to accomplish. First, the current operating cost of the existing lighting system is calculated for a year. In a similar manner, the yearly operating cost for the new lighting system is also estimated. Assuming that the retrofit results in savings, the difference in operating costs between the two is then computed. The (simple) Payback Period is the time period needed to accumulate the return of the total installation cost of the retrofit system and forms the basis for determining the desirability of a retrofit. This time period is the total investment cost divided by the yearly savings.


Current Operating Cost









This article will only address the computation of the current cost of operating a traditional lighting system using incandescent fixtures. The complete calculation for obtaining Payback Period will be left for a future article.

The elements that are contained in the operating cost are: (1) the electrical costs to operate the lighting equipment, (2) the lamp replacement costs, (3) The air conditioning costs for removal of the heat generated by the lighting instruments and finally, (4) the labor cost of lamp replacement.  In my experience, the cost of lamp replacement need not be included as it is insignificant and is easily handled by studio operating personnel in the course of their normal tour.



A Different Approach

To make the lamp operating costs more realistic, I am going to suggest a different approach in predicting lamp power and life expectancy. In actual use, lamps are not actually operated at rated voltage of 120 volts. Not only is there a slight voltage drop through the distribution system, but the dimmers are generally not operated full. This fact results in a large change in expected lamp life as well as the actual electrical power used. Figure (1) illustrates the relationship between actual /rated voltage and lamp life.



You will note that the lower the operating voltage, the longer the lamp life. Figure (2) shows the accepted formulas that are valid for estimating life expectancy and wattage required as the operating voltage deviates from the rated voltage.

 It is my opinion that these should be utilized in determination of lamp life and wattage for use in obtaining the yearly operating costs of the “old” lighting system.


Illustrative Example


As an example of the results of this method, let us say that on our lighting console, a channel slider is about 75.  Assuming a typical “square law” console, a scale reading of 75 would probably operate the lamps at about 100 volts and would deliver  about 60 percent of the light obtained at “full”. (The Color Temperature would be about 3000degK rather than 3200degK, not really a problem). Substituting this voltage value in the formula shows that the life would be extended more than ten times! Applying this operating voltage in the wattage formula results in a power usage of about 75 percent of what would be required at rated voltage. To be on the conservative side, 108 volts would seem to be a good arbitrary value to use in the calculation rather than going the tedious procedure of documenting each fixture’s level. This would correspond to a slider position of about 85, a good compromise average. As a result, the lamp life would be extended about four times and the power required would be about 85 percent of the rated lamp power.

That's Life


As an example, an EGT lamp, used in a 1000 watt fresnel spotlight, is rated at 250 hours. Let’s say that our studio is heavily utilized, about 3000 hours per year. Using the rated lamp life we would theoretically have twelve lamp replacements per year. Using the more pragmatic approach would give the more realistic replacement quantity of three. As far as the lamp replacement cost to be used, I would suggest actual “street” price rather than lamp list price.


Air Conditioning and Heat Removal


There can be significant saving in air conditioning requirements as it is directly related to the efficiency of a the lighting source.  In the case of an incandescent lamp the amount of heat that must be removed is generally accepted as equal to 90 percent of the input power. In our case, this would actual be the product of 85 percent of 90 percent or (1000)(0.90)(0.85) or 765 watts in the case of the EGT.  The final calculation of energy  required to remove the heat generated from the lighting system from should also take into account the weather zone of the facility to account for anticipated number of cooling days.

Final Calculation of Operating Costs


The final step should be obvious at this point. Spreadsheet calculation would seem to be most direct manner to create the list of all our studio equipment, estimate studio usage, apply necessary factors, and then calculate our yearly operating cost. As mentioned, this will be covered at a later article.


The Disturbing Problem


It is anticipated that biggest issue in the retrofit process will be finding substitute fixtures. Performance is the key consideration. It is my opinion that a complete selection of solid state fixtures does not exist that duplicates the entire selection of traditional instruments based upon filament sources. Although large aperture soft light replacements are numerous, a full range of spotlights is not available. If your complement needs the performance characteristics of a spotlight greater than 650 watts, there is no equivalent LED fixture presently offered. It is hoped that time will change this dilemma.



Bill Klages would like to extend an invitation to all the lighting people out there to give him your thoughts at billklages@@@roadrunner.com