- Scovill Units - a measure of the hotness of chili
- Troy Ounces and Pounds - measures of precious metals and gems
- Brinell, Rockwell & Vickers Units - measures of hardness of a substance
- Carats - size/weight of jewels (Not to be confused with Joules - a measure of heat)
- Elasticity, or an object's amount of stretch or bounce
- Dust or pollen particles in the air or on the table
- Smell (odor) intensity and quality
- Light (and I don't mean low-calorie, though that might be interesting too!)
I have a personal interest in this because I'm getting old and our economy is stressing conservation. This sets up a conflict because energy-saving bulbs are dimmer than the old ones, and I feel like I'm walking around in the dusk all the time.
To start off, here are some simplified definitions:
- Light is radiant energy emitted from a source and visible to the naked eye.
- Lumens describe the "volume" of visible light energy emitted over a period of time. One watt of power at 555 nano-meters (a very small wave frequency) equals 680 lumens.
- Luminous intensity describes the lumens coming from a point, measured in candelas.
- Luminance is the amount of lumens projected onto a surface, like a screen, measured in candelas per square meter.
- Illuminance is the quantity of lumens on a defined surface area. The unit is lux or footcandle. A flashlight aimed directly at a surface makes a round spot of X lumens, and when tilted makes a larger oval spot with the same total amount of lumens spread over the larger spot. The lux in the oval spot will be less than in the first round spot because the energy is spread more widely.
- Brightness is a subjective discription ranging from dim to bright. This is how our eyes see (or don't). There are no real units here - just Wow that sun is bright! or Can you turn up the lights?
- A photon is a type of torpedo used in the Star Trek series.
On a bright sunny day we can measure 100,000 lux or 10,000 foot-candles of luminance. On a cloudy day the light is only 1/3 to 1/10 as intense.
The chart shows situations and their measures of lux.
Cats are often credited with the ability to see in the dark. That's not quite true, but they can see with only 1/6th or 1/7th the amount of light that we require.
Now as to why the room appears darker to me - it seems that as we age, the size of our pupils naturally decreases, until by the age of 60 they are 1/3 the size as when we were 20.
This has implications for people who wear contact lenses, or have eye surgeries, or use a telescope.
Apparently this contraction in size happens steadily, across all eye colors, degrees of correction, and in both men and women. It has a name - senile miosis.
Great. People keep telling me I'm senile. I guess they are right!
PS - Have you ever heard of a unit of measure of beauty? Since the ancient Greek woman Helen of Troy was supposed to be so beautiful that her face "launched a thousand ships," a unit of female attractiveness is called the mille-helen.