Architectural Model and Daylight Factor Meters

Megatron closed down operations at the end of February 2010. OTC provides these Megatron pages for information only.


Megatron AML/DFM

Megatron AML, DFM, AML/DFM & Digital AML Light Meters

These light meters have been developed for use in existing buildings and in the design of new buildings with architectural models under artificial skies. By improving the design of buildings and the use made of natural light, significant savings can be made in the electricity used for lighting. The daylight factor is the illuminance on a horizontal surface inside a building expressed as a percentage of the illuminance on a horizontal surface under an unobstructed grey sky; it is a measure of how bright or gloomy the inside of a building will generally appear, when naturally lit. For many situations, the CIBSE Code for Interior Lighting suggests a Daylight Factor of five per cent or more for a well lit appearance, whilst a factor of less than two per cent does not look well lit and electric light will be in use usually throughout the day. These instruments will be of interest to lighting engineers, architects, energy saving consultants, as well as teaching faculties - in fact all those concerned with improving the design and lighting of buildings.

Model Reference

Light Ranges




Digital AML


10 000 lux
1 000 lux
100 lux
In cell
400 lux
200 lux
100 lux
In cells
1 000 lux
500 lux
250 lux
  Daylight Factors
5 per cent
10 per cent
20 per cent
Out cell
100 000
10 000
1 000
Directional cell
50 000 cd/m²
20 000 cd/m²
5 000 cd/m²
Weight - Kg
- lb-oz
5.1 + 2.2*
11.4 + 4.14*
Illuminated Scale  
Scale lenght - mm
- in

Dimensions H x W x L H x W x L
- mm
- in
150 x 330 x 360
6 x 13 x 14½
*75 x 185 x 275
*3 x 7½ x 10¾
Length of leads AM L DFM AML/DFM
ft - in
In Cells
Out Cells

*matching accessory case for the AML/DFM only.


AML- The Architectural Model Light Meter is for use with small scale architectural models generally in conjunction with artificial skies, but in some cases placed outside under the actual conditions expected for the building. It allows twelve illuminance measurements to be made almost simultaneously, and so a number of Daylight Factors can be calculated from these measurements. The light meter has 12 Megatron Type M potted photocells in flat cosine correcting 21 mm diameter mounts The Type M photocells have colour sensitivity approaching that of the CIE observer.

The Instrument is very convenient and easy to use, with only 3 rotary switches. One switch is the standard "On/Off/Battery Check" switch. The second switch selects one of the 3 light ranges. The third switch has twelve positions to select any of the twelve numbered photocells.

DFM - The B.R.S, Daylight Factor Meter was developed with the Building Research Station. The Daylight Factor is used to overcome the problem of the extreme variability of natural lighting. In an attempt to use an ordinary lightmeter to measure the Daylight Factor there will be an inevitable delay between taking the illuminance measurements inside the building and those outside; it is probable that there will have been a significant change in conditions in this interval leading to inaccuracies. The DFM allows the readings to be taken, and if necessary repeated, without delay by switching from the outside to the inside; it is also convenient to use, as the Daylight Factor can be read directly in per cent. The illuminance out-of-doors can either be measured directly or it can be derived from the average luminance of a densely overcast sky at an angle of altitude of 42 above the horizon, known to bear a constant relation to the illuminance produced on a horizontal surface by the whole sky.

The instrument is designed to make these measurements:

  • Daylight Factor (per cent)
  • Illuminance (lux)
  • Luminance (Cd/m²)
The light meter uses three Type M photo-electric cells having colour sensitivity close to that of the CIE standard observer as follows:
the In cell for measuring indoor illuminance, the Out cell for measuring illuminance outside, an alternative directional Out cell mounted in a tube to restrict the angle of acceptance, and a quadrant so that it can be pointed at 42° to the horizontal for outside luminance measurements;
this alternative directional cell is for use when an uninterrupted grey sky is not readily accessible.

The instrument is simple to use with six controls as follows:

    1. The standard "On/Off/Battery Check" switch
    2. The Daylight Factor percent selector switch (0-5, 0-10, 0-20 percent)
    3. A toggle switch allowing the In or Out cell to be selected.
    4. The range switch which also has a Daylight Factor position.
    5&6. Coarse and Fine controls used for obtaining a maximum meter reading with the Out photocell before switching to the In cell to read off the Daylight Factor.

AML/DFM - The Architectural Model Daylight Factor Meter - combines many of the advantages of the AML with those of the DFM; it is very versatile but remains easy to use in practice.

With this instrument the illuminance can be measured at 12 points almost simultaneously (similarly to the AML). As in the DFM, there is a choice of two Out cells for checking the exterior light: a 25mm Type M cell for use when there is an uninterrupted grey sky, and an alternative directional cell with a narrow acceptance angle for use when this uninterrupted grey sky cannot readily be obtained. The use of either of these Out cells allows immediate switching to any of the twelve 16mm cells which then act as In cells; the Daylight Factor is read directly in per cent. Apart from the standard ON/OFF switch, there are six controls as follows: per cent, toggle and range switches, coarse and fine controls similar to those on the DFM, and a selector switch as on the AML with one position for each of the twelve In cells.

Digital AML is for use with small-scale architectural models generally in conjunction with artificial skies. It has an option of Digital Interface and Software to calculate and log daylight factors (2 channels only). It allows 10 illuminance measurements to be made simultaneously from inside a model and 1measurement from a cell placed outside - and so a number of daylight factors can be calculated. The instrument has 11 Megatron Type M photocells in 21mm diameter flat cosine correcting mounts. The ranges are calibrated to suit the user: e.g. 1 to 10 cells at 1,000 lux and 11th cell to 3,000 lux.

Price List

megatron - for measuring light