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.