Fixture photometrics

[Ted Malachowski]

I have a question pertaining to photometrics. I recently installed some Vista uplights and I was comparing the lighting effect between two different fixtures, both of which used the same MR-16 bulb. It seems interesting to me that Vista provides photometric information according to the bulb, when there is a definite effect of placing the same bulb in two different fixtures. Obviously, if one fixture places the bulb much deeper into its housing, the light spread will be greatly reduced. In this case, I see no information pertaining to how the light is effected in the different housings. Do manufacturers typically quantify the effect a given fixture has on the bulb photometrics, so a more accurate comparison can be made of the resulting light output?

[sitelights]

Let me begin by stating that all MR lamps should be specified with "FG" or "C" i.e. front glass or covered. The bonded cover glass prevents deterioration of the mirror surface of the reflector due to condensation. Never use a bare MR lamp in outdoor lighting applications and if you purchase a lamp with a fixture specify front glass or (if the option is not available) purchase the fixture without a lamp and use only covered MR lamps.

The MR lamp and other reflector lamps such as R and PAR types exhibit photometric performance based solely on the lamp and not on the lampholder (luminaire). The beam pattern is independent of the fixture it is placed into. While there are some differences in performance between one manufacturer's product and another, the ANSI codes (BAB, ESX, EXT and so on) usually equate to identical watt and beam no matter who made the lamp. The BAB 20w 35 degree lamp is also 40 degrees from some makers; a 20w 60 degree lamp is also available but as far as I know does not have an ANSI designation. There are subtle differences in terms of filament orientation: some lamps have a vertical filament (preferred) and some are horizontal. It is the relationship between the filament and the facets of the mirror reflector that produce the different beam patterns. Some luminaires regress the face of the lamp into the fixture body and the expectation is that this will produce "beam clipping". In fact, the beam can pass through an opening much smaller than you would expect and this is due to the focal point of the beam (picture an X) occuring some inches beyond the lamp face. This is why "pinhole" trims on recessed ceiling lights do not "clip" the beam.

In fixtures lamped with a non-reflector lamp it is the luminaire's reflector that produces the photometrics in combination with the lamp's wattage and the reflector's characteristics. In this instance it is the performance of the lamp plus the performance of the reflector that produce the (highly variable) photometrics. Since the photometric charts assume a beam on a light or white surface actual performance is more a function of site conditions (what is being lighted and from what angle) than numbers on a chart.

I suspect that variations in beam pattern using identical lamps in different fixtures is simply a function of site conditions. On a damp or foggy night the beam patterns will prove to be identical as seen in the open air.

[Eden Lights]

While I understand the reply, I have to say that I have played with fixtures with different lense depths, glare shield shapes, sizes, and etc. and have found big differences in the quality of photometrics in first few feet of MR16's lamps, smoother light patterns and even different widths of photometrics in the first few feet of throw. When it comes to say grazing a wall with min. setback, I have to say I have favorite fixtures and BAB's.

[sitelights]

Beam characteristics of MR lamps vary widely; what is measured is beam width and footcandle levels under ideal conditions. ANSI codes provide uniform nomenclature relative to these two measurements to facilitate equivalent lamp replacement. These are the quantitative elements provided by photometric tables. The qualitative elements such as beam smoothness, striation, notching, hot spots and uniformity are not measured but are left to the advertising department to tout as features that differentiate the aesthetics of lamp performance. Note that the simplified photometric charts illustrate results in two dimensions while sophisticated charts show the beam as round or, dependent on aiming angle, ovals of varying dimensions. I stated in an earlier post that photometrics are measured by illuminating a white surface and it is fairly simple to measure beam size at different distances from the point source. Footcandle measurements can also be made but this takes rather specialized equipment. Even with a footcandle meter (see "light meter" under product reviews) of known and calibrated accuracy it may not be possible to duplicate the manufacturer's data. Without measurement one merely has a subjective opinion but that is enough. Low voltage outdoor lighting seldom illuminates a flat white wall and instead lights very complex (in terms of multiple planes, angles and degrees of reflectivity) surfaces. In fine art lighting for museums and galleries the effects are a combination of the quantitative and qualitative aspects of lamp performance and it is possible under these controlled conditions to achieve perfect lighting. In an outdoor setting with a full set of variables to contend with, perfection is neither to be attained nor sought. Certainly there are variations in the manufacturing tolerances of mass produced MR lamps that can result in minor differences in performance from batch to batch. As an example, MR lamps used in medical apparatus are made to much closer tolerances and their cost reflects this. I do maintain that the subjective perception of beam shape is clearly validated on a foggy or rainy night when the beam can be seen in the open air. Under these conditions the observed performance of a single lamp type in a number of different luminaires should be about equal if aiming angles are identical.

My point is simply that within the parameters of outdoor lighting precise effects are limited by the quantitative aspects of lamp performance; the quality of the beam as outlined above is not an issue except in terms of the "whiteness" (read: voltage at the lamp) of the projected light.

In terms of wall grazing, I much prefer to use a floodlight with a 120+ degree beam spread since this provides greater dispersion with a trapeziodal rather than inverted cone shaped beam. Also, the 60 degree 20w MR-16 lamp (Ushio's Superflood) has a real advantage over the BAB lamp in such applications.

[Ted Malachowski]

I have another question related to beam spread, which has come up as I finalize the landscape lighting in my front yard. I placed two Vista GR2216 bullets with LN20-BAB MR16 bulbs directed at the two posts which support the small overhang in front of my front door. One bullet per post, about two feet from the base of the post. The posts only extend the height of the first floor, and therefore the top of them are only 10' about the bullet. In this position, I'm getting the light I want on the post, but I'm also getting lots of light on the second floor, which I don't want. Obviously this is due to the photometric of the bulb, so I called Vista and spoke to one of their technicians regarding my options. They are:

1. Substitute the LN20-BAB bulb with a narrower one, such as LN20-ESX (I believe thats the number). This bulb has only a 2-3' spread at a height of 10', versus the BAB, which is closer to 7-8' at this height. The drawback to this would be that the light would be extremely bright, since its the same wattage with a narrower beam.

2. Use a 10W MR-16 bulb with the same photometrics as the LN20-ESX. I believe the model number he gave me was LN10-ALR.

3. Install a honeycomb louver below the glass in my current fixture, and continue to use the current LN20-BAB bulb. The technician stated this will dramatically reduce the beam spread, but won't cause an overbright condition, as would be experienced with the LN20-ESX.

I'm looking for any advice/feedback. Thanks again for all the replies.

[sitelights]

There is no reprieve when you transgress on the Laws of Optics; you can't stop light without a barrier. The "overhang" is the barrier but the aiming angle 2':10' allows light to project beyond the small overhang. If you placed the uplights tight to the posts with the shrouds oriented away from the posts the overhang may be deep enough to stop the dispersion of light above that plane. Experiment by tilting the shrouds toward the posts at night until the angle of the shrouds contain the beam within the lip of the overhang. Yes, the posts will appear "hot" at the beam center point (about 7' up the posts...a guess) if the posts are white or a light color but Vista's suggestions about 10w MR-16s and/or hex louvers might alleviate that. This is what I call fine-tuning and it can only be done at night. Another possible solution is simply to increase the overhang but now we've got a carpenter on site.