Lighting Instruments:
Ellipsoidal Reflector Spotlights


      J. Michael Gillette. Theatrical Design and Production, 4th edition. Mountain View, CA: Mayfield Publishing Company. 1999. Chapter 14: Lighting Production, pp. 340 - 344.

1. What is the difference between a floodlight and a spotlight?

A floodlight produces a large, almost uncontrollable wash of light; a spotlight, on the other hand, produces a small (8' to 12' diameter), highly controlled pool of light. Floodlights are generally used to light the backdrop while spotlights are used to light the actor.

2. What instrument is normally used to light the actor?


    The scenery?


3. List six traditional lighting instruments which are commonly used on the modern stage and indicate which are spotlights and which are floodlights.

  1. Ellipsoidal Reflector Spotlight -- Spotlight
  2. Fresnel Lens Spotlight -- Spotlight
  3. PARcan -- Spotlight
  4. Follow Spot -- Spotlight
  5. Automated Fixtures -- Spotlight
  6. Ellipsoidal Reflector Floodlight (Scoop) -- Floodlight
  7. Border Light -- Floodlight

4. Evaluate each of these instruments by its beam angle, spill and edge quality.

Instrument Beam Angle Spill Edge Quality
Ellipsoidal Fixed Very little Hard
Fresnel Variable Quite a bit Soft
PARCan Fixed Some Soft
Scoop Fixed A lot Soft
Border Light Fixed A lot Soft
Follow Spot Variable Almost none Hard

Ellipsoidal Reflector Spotlight

Source 4- 26°
6" Ellipsoidal Reflector Spotlight
The six inch, Ellipsoidal Reflector Spotlight, often referred to as an Ellipsoidal, Leko, (Pronounced: Lee-kO), ERS or Profile Spot, is the work horse of the commercial theatre. The instrument produces a hard edged, highly controlled pool of light. In academic and community theatres it is primarily found Front-Of-House in the Ceiling Cove, and on the Balcony Rail and Box Booms. Most commercial productions are lit almost entirely with the ETC Source 4 Ellipsoidals. The size of the pool of light is determined by the throw distance and which of the nine available lens chains the designer chooses. The Ellipsoidal has traditionally been specified by either the lens system (6x9, for example) or the field angle (36°) (Cost of an ETC Source 4 Ellipsoidal (50° to 19°) with a 575w lamp: $ 386 at StageTechnology)

ETC Source 4 Ellipsoidal Reflector Spotlight
at 40' throw
Factor (MF)
with 750w HPL
90° 75' 1.88 7 fc @ 40' throw
70° 56' 1.4 14 fc @ 40' throw
50° 38' .95 22 fc @ 40' throw
36° 25' .63 51 fc @ 40' throw
26° 18' .45 99 fc @ 40' throw
19° 13' .32 180 fc @ 40' throw
14° 10.4' .26 265 fc @ 40' throw
10° 8' .19 523 fc @ 40' throw
5' .12 854 fc @ 40' throw

Multiplication Factor = Diameter of pool / Throw distance
Diameter of pool = Throw distance * Multiplication Factor

    What is the difference between an instruments beam angle and its field angle?

The Beam Angle is the angle of spread where the intensity of the light drops to 50% of the mazimum intensity at the center of the beam. The Field Angle is the angle of spread where the intensity of the light drops to 10% of the mazimum intensity at the center of the beam. It is generally the Field Angle which is published in the manufacturer's specs.

    Sketch the standard light plot symbols for the original six Source 4 Ellipsoidals.

5. What type of lens system is used in an Ellipsodal Reflector Spotlight?

The conventional Ellipsoidal (an Altman 360Q, for example) uses a lens chain composed of two plano-convex lenses arranged in a belly-to-belly configuration. Lenses are specified by their diameter and focal length. A 6x9 lens is 6 inches in diameter and has a 9 inch focal length. With the exception of the 36° unit, the Source 4 "6 inch" Ellipsoidals all use a single double-convex lens. ETC uses the lens notation after the field angle in their Source 4 specification as a reference to a conventional (Altman 360Q) ellipsoidal. They do not indicate the lens system ETC is using in the instrument.

6. What type of reflector?

An ellipsoidal shaped reflector. An ellipse has two focal points. The filament of the lamp is placed at one focal point and the shutters or gobo is placed near the second. The second focal point is also the focal point of the lens chain.

7. Which instrument, a 36° (6x9) or a 26° (6x12), should be used for a longer throw?

The 26° (6x12) unit. Typically the longer throw requires the narrower beam angle. I use a 36° unit on my on-stage electrics because the maximum thow, pipe to floor, is about 24 feet. I use 26° lamps in the Cove because the throw to the head of an actor standing at the Curtain Line is 40 feet.

Some designers use a table, like the one below, to determine which Ellipsoidal to use for a given throw distance. The Minimum Throw produces a field diameter of about 8'. The Maximum Throw drops the intensity to approximately 50 foot candles with a 575 watt lamp.

Throw distance °Ellipsoidal
Less than 20' 50° ERS
15 to 30' 36° ERS
20 to 40' 26° ERS
30 to 55' 19° ERS

8. What range (wattage) of lamps are used in the 6" Ellipsoidal?

The traditional Ellipsoidal was designed for the 500, 750, and 1000 watt lamp. The Source 4 ERSs designed by ETC use a specially created and highly efficient 575 or 750 watt lamp.

9. How large a pool of light would be produced by a 26°(6x12) Ellipsoidal with a 20' throw?

The field diameter would be about 9 1/2'.
There are three ways to determine the size of a pool of light--
  1. Graphically with a protractor, straight edge and scale (or a CAD program),
  2. Mathematical: Size of pool = multiplication factor * throw distance, or with a
  3. Computer software program such as John McKernon's BeamWright5, a Freeware program which can be downloaded from John McKernon Software.
Using the mathematical approach: 20 foot throw * .45 multiplication factor (for a 26) = 9.4 feet

10. If the intensity of a 26° (6x12) Ellipsoidal is 95 foot candles with a 40' throw, what would be the intensity of the light with a 20' throw?

First, establish the Beam Candle Power (BCP) of the instrument by multiplying the intensity (95 fc @ 40') times the throw distance (40') squared. The BCP = 95 * 40 * 40 = 152,000 bcp.
Then divide the BCP (152,000 bcp) by the square of the new throw distance (20'). The intensity of the light = 152,000 bcp / 400 (the 20' throw squared) = 380 foot candles.

Below is a simple JavaScript program which will calculate
Field Diameter and Center Intensity for a ETC Source4 ERS.

(For a "Straight Throw" such as Down Light or "Straight In" Front Light from the Balcony Rail)
Choose the Instrument, Enter the Throw Distance and Click the Calculate button.

Choose Instrument:

Enter Throw Distance: feet.

Field diameter = feet.

Center Intensity = foot candles with a 575 watt HPL lamp.

Link to Sean Hennessy's BeamCalc, an online calculator similar to John McKernon's BeamWright.

      What is "wash spacing?"

Both Sean Hennessy and John McKernon calculate a "wash spacing," the distance between two lamps to produce an even blend (or wash) of light. According to Hennessy's BeamCalc, a Source4-36° with a 24' throw produces a Field diameter of 14', a Beam diameter of 9 1/2' and a Wash spacing of almost 12'. So if I want a wash of down light from the first pipe, I would hang a lamp on the center line, and additional lamps at 12' and 24' left and right of the center line. Steve Shelly's formula in A Practical Guide to Stage Lighting is much simpler: Wash spacing = 1/2 of the Field diameter.

11. How do you shape the pattern of light from an Ellipsoidal?

You can shape (actually frame or square-off) the pattern of light with the four framing shutters.

12. What is a "gobo" pattern?

A gobo is a pattern (or template) which can be dropped into the "gate" of an Ellipsoidal creating a crude slide projector. A gobo can be used to project either a realistic image, such as a cloud pattern, or simply create a textured light.

13. What is a Zoom Ellipsoidal?

Although the Ellipsoidal generally has a fixed beam angle, Strand, ETC and Altman each manufacturer a pair of Zoom Ellipsoidals. ETC produces two zoom ellipsoidals. One zooms from 15° to 30° and the second unit zooms from 25° to 50°. The cost of either ETC Source 4 Zooms, with a 575 watt lamp, is $ 570 at StageTechnology -- $ 184 more than the fixed beam unit. In addition to being considerably more expensive, a Zoom Ellipsoidal is both longer (23") and heavier (17 pounds) than its fixed beam angle brother.

14. In what mounting position are Ellipsoidals traditionally hung?

Front-of-house-- Balcony rail, Cove, Box Booms, FOH Pipe, Booth Pipe and anywhere back stage where the need to shutter the light off of the scenery (or out of the auditorium) is essential.

E-mail questions and comments to Larry Wild at
Revised: January 6, 2011
Copyright © 2001-2011 by Larry Wild, Northern State University , Aberdeen, SD