Light Meter

A light meter is a device used in photography to measure the brightness of a scene and thus determine the optimum exposure of a shot.

This tool measures the illumination (amount of light received in lux) at a defined location (usually where the subject is) irrespective of the reflectivity of the subject itself and calculates the optimum aperture or shutter speed.

They should be distinguished from flash meters designed to measure the light emitted by flash units.

There are two distinct ways to measure light to ensure a good exposure to our shots:

• Measuring incident light

It is the amount of light emitted by the light source (sun, spot, ...). To measure it, the brightness sensor (located on the side of the screen) must be placed in the light source (taking care not to interfere between the screen and the light source) and click on the button (Or volume +/-) to freeze the amount of light received.

• Measurement of reflected light

It is the amount of incident light reflected by the subject. To measure it, place the brightness sensor (located on the side of the screen) towards the subject (close enough 20/30 cm) and click the measurement button (or volume +/-) to freeze the quantity of reflected light. Since the measurement varies according to the color and / or reflection coefficient of the subject, place a white sheet on the subject and orient the screen towards the subject at an angle of about 45 °.

Measuring incident light

Advantages:

• High accuracy of measurement
• Identical measurement irrespective of the color and / or reflection coefficient of the subject
• Possibility to make a whole series of photo under the same light without ever changing its exposure

The defects:

• It is necessary to be physically close to the subject or for subjects illuminated by the sunlight placed in an area receiving the same intensity of light
• The incident light measurement does not take into account a possible colored, polarizing or other filters
• The incident light measurement does not take into account the elongation of the photo macro shot or a focal multiplier
• It is necessary to press each time to make a new measurement and to report the figures in manual mode

Measurement in reflected light

Advantages:

• High speed - measurement and adjustments are almost instantaneous because the meter is integrated in the cameras.
• The built-in meter takes into account the filters you mount on your lens, as well as the lengthening of the macro print or the presence of a focal multiplier

The defects:

• The measurement varies according to the color and / or the reflection coefficient of the subject, doing the work of automatic changing and finally relatively unreliable
• Less precise overall

If your device doesn't have a light sensor, you can use the camera instead. The measurement will be done by taking a photo and examining the EXIF data (no real time measurements in this case).

Use

The tool allows you to either determine:

• the aperture according to the speed and ISO chosen (mode 1)
• the speed according to the aperture and the ISO chosen (mode 2)

Mode 1

1. Unlock the speed by clicking on the ? (if it is positioned on the speed)

2. Set the speed and ISO you want

3. Orient the phone either to the light source (incident light) or to the subject (reflected light) and freeze the brightness value by clicking the on-screen button or the volume +/- buttons

In the bottom area

4. You get:

• The aperture corresponding to the brightness
• The brightness found
• The color temperature (if your device has an RGB light sensor)

5. You can transfer this data to the equivalent exposure calculation tool

Mode 2

1. Unlock the aperture by clicking on the ? (if it's positioned on the aperture)

2. Set the aperture and ISO you want

3. Orient the phone either to the light source (incident light) or to the subject (reflected light) and freeze the brightness value by clicking the on-screen button or the volume +/- buttons

In the bottom area

4. You get:

• The speed corresponding to the brightness
• The brightness found
• The color temperature (if your device has an RGB light sensor)

5. You can transfer this data to the equivalent exposure calculation tool

Calibration

The result obtained by this brightness sensor may be different from reality and it is for this reason that it must be calibrated so that the result display is the closest to reality.

Principle:

• Take a sheet of standard matte white paper, fix it on the surface of which you want to know the illumination (flat surface or almost). If the light has a majority direction, try placing the sheet in a plane perpendicular to the direction of that light.
• To avoid disturbing the measurement with reflected light:
  1. place a dark surface on the plane whose illumination is to be measured
  2. place a white sheet not too big (A4 sheet folded into 4)
• With the camera, take the sheet as the aiming target, paying attention
  1. that the blank sheet occupies 100% of the field of view,
  2. not to shade the target,
  3. the angle of view with the sheet is as close as possible to 45 °
• Record the 3 exposure values ​​suggested by the device (ISO, shutter speed and aperture) as necessary redo the operation 2 or 3 times to ensure that the values ​​are stable.
• Report these values ​​in the formula and calculate: L = 48 x Focal² / (ISO x Shutter Speed)

-> you obtain a fairly correct value of the illuminance in lux of the target area.

• Measure the incident light with your phone and adjust the result (calibration) to get close to the value obtained with the formula.

Do not expect much accuracy with the phone's brightness sensor. A difference of 50% can be obtained with the value given by a good luxmeter.

As with a luxmeter, to take a measurement, it is advisable to place the phone on the surface whose illumination is to be measured and to position it in front of the (main) light.

Reflectance (or Albedo)

In most cases, the light that will arrive on the sensor (or film) will be the reflected light. Each surface has a different reflective power, called reflectance (or albedo). It's possible to change this reflectance for calculations in measurement of the incident light. By default this value is 12.5%.

If you want to enter a value that does not exist in the list, simply make a long click on the drop-down list and enter its value.

Example of reflectance values:

Ground, sand, rock

• Carbon (Coal, forged iron): 4%
• Black soil (dry): 7% - 8%
• Black soil (wet): 2% - 5%
• Bare soil: 17%
• Grey soil (wet): 10% - 12%
• Grey soil (dry): 25% - 30%
• Dry silt loam soil: 23% - 28%
• Dry clay soil: 15% - 35%
• Dry sandy soil: 25% - 45%
• Dry salt cover: 50%
• Sand (dry): 40%
• Sand (wet): 23%
• Yellow clay: 16%
• Rock: 30% - 40%

Vegetation

• Conifer Forest: 8% - 12%
• Deciduous trees: 15% - 18%
• Tall wild grass: 16% - 18%
• Short green grass: 20% - 25%
• Grain crops: 10% - 25%
• Corn field: 16% - 17%
• Tea bushes: 16% - 18%
• Summer foliage: 9% - 12%
• Autumn foliage: 15% - 30%
• Tundra: 20%

Snow, ice, water

• Melting snow (clean): 60% - 62%
• Ice: 50% - 70%
• Ice (sea): 30% - 45%
• Ice (glacier): 20% - 40%
• Fresh snow: 70% - 90%
• Water (sun near zenith): 50%
• Water (sun near horizon): 50% - 80%

Asphalt

• New asphalt: 4% - 5%
• Aged asphalt: 10% - 18%
• Wet asphalt: 6% - 8%
• Gravel: 13%

Roofs

• Tar & gravel: 33%
• Corrugated roof: 10% - 15%
• Red/Brown roof tiles: 10% - 35%
• White asphalt shingle: 20%

Concrete

• Aged concrete: 20% - 30%
• New concrete (traditional): 40% - 55%
• Polished concrete: 55%
• New concrete with white portland cement: 70% - 80%

Wood

• Batten (fresh wood): 35% - 42%
• Batten (old, weathered): 12% - 16%
• Varnished wood: 13%

Tiles, bricks, finishings, etc

• Terracotta tile: 28%
• Brick: 20% - 40%
• Sandstone: 18%
• Plaster: 40% - 45%
• Magnesium oxide: 96%
• White gypsum: 85%
• Alabaster: 92%
• Granite gray: 35% - 40%

Paint

• Black acrylic paint: 5%
• White acrylic paint: 80%
• Colored paint: 15% - 35%

Misc

• Forged Iron: 4%
• Natural silk fabric: 35% - 55%
• Skin: 25% - 35%
• White paper sheet: 60% - 70%
• Mirror: 100%