My school has all Altman 360 lights. Not 360Q. They're really terrible lights, but anyway... I'm trying to distinguish between the 6x9 and 6x12. My guess right now is that the 6x9 is the left fixture, and the 6x12 is the right fixture. Coke can and tape measure is for scale. I'm trying to replace these with lights that have similar beam spread.

Yeah it looks like a 6X9 and a 6X12. Shine them at a surface 10 feet away, a 6X9 should have give you a circle of light about 6 feet in diameter while a 6X12 should be about 5 feet. The better way to approach replacement with an appropriate instrument is to do some math. But, before we continue we need a quick lesson on photometrics. The two terms you need to understand are Beam Angle and Field Angle. To understand these we are going to imagine a giant bull’s-eye drawn on a wall. You shine an instrument straight at it's center so that you get a perfect circle. The intensity is brightest in the middle and fades as you reach the outer edges. If you were to measure the intensity of light at each circle you would find a point toward the outside edge where the intensity of light is 1/2 of the intensity at the center. The angle of the cone of light spreading out from the front of the lighting instrument to this ring on the Bull's-eye is called the Beam Angle. If you continue measuring light intensity you will find a point where the light is 1/10 the intensity of the center of the bull’s-eye. The angle of the cone of light spreading out from the front of the lighting instrument to this ring on the Bull's-eye is the Field Angle. The light inside the beam angle circle is really good light. The donut ring of light between the beam angle ring and the field angle ring is useful, but this is where you will need to overlap with other instruments some to keep the wash even. The small amount of light beyond the field angle circle is pretty much useless. Now back to your needs. First either measure or do some geometry to figure out how far of a throw distance you are trying to cover. Now find the specs or photometrics for the specific instrument you are interested in purchasing and do some math. I've gone to ETC's website and gotten the numbers for Source4's to teach you how. But other manufacturers as well as your local lighting dealer will have similar information so check their websites. On ETC's website they give you a multiplying factor. The number multiplied by the throw distance gives you the diameter of the circle of light. There is a different number to calculate the beam diameter and the field diameter. Source4 multipliers: 50 Degree: Field Diameter .95 Beam Diameter .60 36 Degree: Field Diameter .63 Beam Diameter . 45 26 degree: Field Diameter .45 Beam Diameter .33 19 degree: Field Diameter .31 Beam Diameter .27 10 degree: Field Diameter .19 Beam Diameter .16 5 degree: Field Diameter .12 Beam Diameter .11 SO, if your 36 degree S4 is 20 feet above the stage multiply and you get: Beam diameter ring of: 9 feet and a Field diameter ring of: 12.6 feet That's just about right. I always try to keep my field diameters around 12 to 17 feet.

Using the measuring tape you have in the picture here are the minimum lengths of the altman fixtures with lens completely retracted. 6 X 9 - 16 1/2" 6 X 12 - 20 1/8" 6 X 16 - 21" 6 X 22 - 29"

Are you sure that's right? My understanding is that Altman didn't make the 360 bigger than 6x12. That seems to be for the 360Q.

these are 360 not 360Q fixtures and also you are correct at least in the snout lenses installed in the fixtures. As to what lenses are in the fixtures, anyone's guess but one would assume that a 6x9 snout has a set of 6x9 lenses in it. Bench focus and verify the beam as mentioned above, but at least by way of other than lens, yep, you are correct.

Ship, Yes, the measurements I listed are for the 360Q and not the 360 but the relative difference between the measurements will remain the same as the barrel assemblies did not change between the 360 and 360Q. Only the lamp house changed.

No no, you misunderstand me. Your info was good to post in providing lengths fixtures but length of barrel would probably be better here. I was only confirming by looking at them that the 6x9 was on the left. Also I thought I read in the origional post or something like that an assumption that they were 360Q fixtures. Overall I was responding to and confirming AVGuyAndy' origional "I'm trying to distinguish between the 6x9 and 6x12. My guess right now is that the 6x9 is the left fixture, and the 6x12 is the right fixture." I understand and agree that barrels between 360 and 360Q fixtures are for the most part interchangable given mounting holes are often slightly different dependant upon how close to lunch it was for the person making them.

I found dimensions of the 360 on Altman's site. The left one is definately a 6x9, and the other is a 6x12.

The one that look like a 6x9 could also be a 4 1/2, the are identicle other than the barrel, at least rhe 360Q's are

Remove the Lense tube from the unit and find the distance from the floor or table that the light is focused into its smallest beam. Measure the distance from the front lense to the table or floor to find out what type of unit it is. THe 6 means a 6" diameter lense, the 9 or 12 stands for the distance from the unit the light converges.

The barrel should have two 6x6.5 lenses to produce a 6x9 fixture. If only one lens is fitted then it is a 6x9 but many manufacturers use two lenses. The 6x9 means a six inch diameter lens with a focal length of nine inches. The easy way to check a focal length of a lens is by holding the lens above a page with text on it. Hold a twlve inch ruler vertical to the paper. Hold the lens above the paper and move the lens up or down until the text on the paper is in focus when you read it through the lens. Check the height of the lens above the paper using the ruler, the height in inches is the focal length. In case you are wondering how two 6x6.5 lenses give you the equivalent of a 6x9 lens the answer is that two lenses close together behave as a single thick lens the maths for this is summarised in Gulstrand's equation. This is the basis for how a zoom spotlight and follow spot work.

Intentionally necroposting as the above post contradicts much of what I have learned/believed regarding ERS optics. So the "read the text thru the lens" trick didn't work so well for me; I ended up taking some lenses out into the sunlight and did the "burning ants" trick. I didn't have a 6x9 fixture (thought I did, but it turns out the Century 1500 I bought off ebay contained one 6x12 and one 6x16 lens when I took it apart. I've never powered it up as it has no socket.) I measured the lenses of a Strand-6x12 as having focal lengths of ~11". I measured the lenses of a 360Q-6x16 as having focal lengths of ~15". Now very curious, I measured a SourceFour-19° and found a focal length of 8". So I have to think that a standard 6x9 fixture contains two 6x9 lenses, and has an EFL of 4.5" This is further exemplified by my experience years ago when using a Kliegl 1355 (which contained a single 6x8 step lens) and getting comparable performance (size-wise at least) to a 6x16. Still more than open to listening to more discussion, however. If anyone has a 360/360Q and would be willing to take a lens outside and measure it, I would be grateful. Thanks church for prompting me to add EFL and Gullstrand's Equation to the wiki.

I have some 6x9's with me here at the house, but unfortunately the sun is not out here in Texas. I'll try this tomorrow, it's supposed to be a much nicer day.