Now most would think that the light fixture would be chosen first for each element of a lighting project. Well, very early on I realized that I was actually approaching this the wrong way. The priority SHOULD be on identifying the correct light source THEN finding a suitable fixture to hold the light source.
OK, so you may say that one needs an up-light fixture for all the lights on an architectural lighting project that you are working on. Well, yes that may be accurate. BUT, there is a very wide selection of up-light fixtures that are configured to accommodate light sources that provide specific light patterns.
Here are a couple examples to help enforce the point I am making:
Cast Lighting – CID140
With this fixture one can have a very narrow beam, a very wide beam or a rectangular (actually oblong) beam shape. And the intensity may be adjusted at 20 different levels. And it’s price reflects that it is loaded with features that make it flexible.
Cast Lighting – CIWL6
This specialty fixture has a very diffused light with no “hot spot” and a very wide and fairly high pattern of light distribution. The overall brightness level (lumens) ranges from low to medium.
Cast Lighting – CBL5CB
Sold as an empty shell, this fixture has no light source installed from the factory. One must select the light source desired and purchase it separately.
So don’t get hung up on a particular fixture. By starting with the light source requirement FIRST, you are able to focus your attention on the IDEAL light source for each element. Of course, reality will come into play at some point and you will be required to make some compromises based on several factors such as budget, availability and others. That is OK, because your target was the IDEAL light source. In most all cases a light source close to the target is acceptable. And only you will know it was not your target choice.
OK, now that I have worn that idea out, let’s consider light sources. Here are a few baseline assumptions about the light sources: they will be LED, they will be low voltage (12 volts A/C nominal), they will be Warm White (~2600 – ~3200 Kelvin) and rated for outdoor use.
I chose beam angle first. Yes, not brightness (lumen level). Why? Because for a given wattage (or more accurately, lumen level) the effective brightness can vary depending on beam angle. Let’s dig into beam angle a bit so that the terminology used is meaningful.
Useful Terminology
Lumen Output – Total visible light output of a light source. This is typically measured by putting the light source (or complete fixture) in a special sphere that measures ALL the light emitted from the light source no matter what direction the light is traveling.
Center Beam Brightness – Yep, you guessed it, the brightness of the light beam at it’s center. The unit of measure for this is typically candela. One candela is approximately the brightness of one wax candle.
Beam Angle – This is determined from knowing the brightness at the center of the beam and finding the point where the brightness degrades to 50% of the center. Example: In a specific light source the Center Beam Brightness is 500 lux, going outward from the center to a point where the brightness equals 250 lux would mark the edge of the beam angle. If the angle measures 30° from the center of the beam to the 50% brightness point the Beam Angle would be 60°. That is, 30° each side of the beam center for a total 60%.
Field Angle – This is determined from knowing the brightness at the center of the beam and finding the point where the brightness degrades to 10% of the Center Beam Brightness.
I find it very useful to note that BEAM ANGLE is NOT how far out from the center that there will be noticeable light. The light will typically extend well beyond this point, albeit with degraded but potentially useful brightness.
Notepad Sketch of Beam and Field Angle

I find a rough analogy between choosing the width of a paint brush to choosing the width of the beam. Yes, you could chose a wide brush and slather paint over every surface of a structure but, there would be lot’s of wasted paint in areas where you did not want paint. It is similar with light … why waste light? Why not target putting the light where you want it? Do you really want your customers paying for light (electricity) that is illuminating the sky?
Another important point is that you may need to send light a rather long distance to get it from the light source location to an element of your lighting design. So, a narrow beam angle may be necessary in order to have the brightness required at a given distance.
So, how to choose …
The two major factors to consider is the SIZE of the element to be illuminated and the DISTANCE to the element. It really is THAT simple. Now, you can study photometric charts to the nth degree and get information on how bright a light beam will be at a certain distance from the source but, in real life you are somewhat limited to what is available. I am not saying these charts are not useful. On the contrary, they are valuable in comparing the relative brightness of different light sources at a given distance. Especially if you are new to lighting or a lighting product is new to you.
So, to make it easy to get started think in terms of extra narrow, narrow, medium, wide and extra wide beams. As you do your walk-through of the property with your baseline sketch in hand, make notes of where you would like to place a light source, what direction it will be aimed and how wide the beam will need to be. Also note if a beam should ideally be “Xtra” narrow or “Xtra” wide to get the effect you desire.
Helpful Beam Angle Abbreviation Chart
- Extra Narrow – XN
- Narrow – N
- Medium – M
- Wide – W
- Extra Wide – XW
Remember that your major lighting design targets are no “Black Holes” and “Zero Glare”. You are targeting even light coverage without shining lights into windows or placing light sources where they cannot easily be shielded to prevent glare. And you may have one area that you would like to give extra visual attention to and thus need to provide a higher level of brightness.
Guidelines
Here are some examples that will help in understanding how different beam angles may be applied.
Narrow – Corners of tall buildings, mature trees’ trunk lighting, large leaf evergreen trees, accents (such as artwork, boulders, waterfalls), columnar tree forms, areas between windows, flags on tall poles, cupolas, weather vanes, columns, objects at great distances
Medium – Corners of buildings, branches of mature trees, small trees, fine needled evergreen trees, gable peaks, wide waterfalls, area lighting, large boulders, wall lighting
Wide – Wall washing, wide and low branched trees, area lighting, background lighting
The next post will be information on selecting the lumen level (brightness) of a light source.
Disclosure: We are a distributor for Cast Lighting and a dealer for Brilliance LED products along with other lighting equipment suppliers. Since we are familiar with these products we use them as examples in our articles. In addition, Cast Lighting uses independent labs for lighting performance data related to their products. So one can find highly detailed, real world data on the light output of their products which makes for very good examples in my articles. Here is a sample of a test report from a 36° – 250 lumen directional light source.