Measuring Light

Yes, indeed you can. Whilst it may seem impossible because we can't physically touch it, light is in fact measurable just like the temperature of a room.  Though  we would use the term light intensity. If you're one that ever seen a photoshoot go on, you'll see that before the button is pressed to take the winning shot, this little doo hickey appears


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This little contraption is where the magic happens and is called a  light meter

A light meter  has the ability to determine the proper exposure for a photograph, and determines whether a room is too bright or dim for a specific requirement for a photo and measures the intensity of light. Also called illuminance

Illuminance: The luminous flux per unit area at any point on a surface exposed to incident light. It is measured in luxes. 

Exposure:  the amount of light per unit area reaching a electronic image sensor (or film), as determined by shutter speed, lens aperture and scene luminance

Sensing there may be some confusion, we may need to step back and define a few more terms.

Lux: lux is the unit of measurement for light intensity. One lux is equivalent to the light level of a single candle light. The symbol for Lux is lx

ผลการค้นหารูปภาพสำหรับ lux measurement

Hoping that a few definition terms are all cleared up. Let's continue.

Whilst taking photos and using a light meter to measure the illuminance, certain numbers must be kept in might. Found below are examples of the amount of light intensity in various situations.

Condition Illumination
(ftcd) (lux)
Sunlight 10000 107527
Full Daylight 1000 10752
Overcast Day 100 1075
Very Dark Day 10 107
Twilight 1 10.8
Deep Twilight 0.1 1.08
Full Moon 0.01 0.108
Quarter Moon 0.001 0.0108
Starlight 0.0001 0.0011
Overcast Night 0.00001 0.0001
Activity Illumination
(lux, lumen/m2)
Public areas with dark surroundings 20 - 50
Simple orientation for short visits 50 - 100
Working areas where visual tasks are only occasionally performed 100 - 150
Warehouses, Homes, Theaters, Archives 150
Easy Office Work, Classes 250
Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories 500
Supermarkets, Mechanical Workshops, Office Landscapes 750
Normal Drawing Work, Detailed Mechanical Workshops, Operation Theaters 1,000
Detailed Drawing Work, Very Detailed Mechanical Works 1500 - 2000
Performance of visual tasks of low contrast  and very small size for prolonged periods of time 2000 - 5000
Performance of very prolonged and exacting visual tasks 5000 - 10000
Performance of very special visual tasks of extremely low contrast and small size 10000 - 20000

You may wonder, well how on earth would I know all that? Math. There is math in everything and you can calculate it. Though we won't get into that right now.

Now you may wonder, great. I understand the terminology but how does this all come together?

  1. Set your camera to manual mode, in order to set up the ISO settings and aperture settings ( F Settings)       

 ISO:  measures the sensitivity of the image sensor towards light in how much light is let through,  The higher the ISO, the more sensitivity to light. Most of the time, lower ISO settings produce clearer pictures whilst a higher ISO will cause graininess

Aperture : The aperture setting changes the size of the lens, and therefore how much light the camera lets in. This setting is describes using the unit f/stops. A larger aperture number, like f/11, means a smaller lens size, and a smaller number, like f/1.4, means a larger lens size. Aperture affects your photos' depth of field and shutter speed

2. Input the ISO number and aperture into the light meter. Replicate the settings on your camera into the light meter.

3. Turn on the light sensor

4. Set your light meter to the appropriate mode . Depending on whether you will use flash or not

5. Aim the light meter towards the subject that you are taking a photo of. Then press the button on the light meter.

Once the measurement is done by the light meter, it will give you the appropriate ISO and Aperture settings in order to capture a picture in the given lighting setup.