iLightMeter is a reflected light meter for iPhone 3gs, iPhone 4 and iPod Touch 4G.

iLightMeter measures exposure with f-number or speed priority, averaging the whole scene or in spot mode. It Includes CINE mode to visualize shutter frame rates. The dEV mode measure the under/over exposure with respect to the selected speed and f-number.
ISO, f-number and speed ranges are:
ISO: from 6 to 3200 in step of 1/3EV
f-number: from f1 to f64 in step of 1/3EV
Speed: from 1/8000 to 30" (from 2f/s to 360f/s in CINE mode) in step of 1EV

BUTTONS
ISO/CINE: Tap and release to set the ISO value. The ISO text on the display blinks, and the + and - buttons are used to set the desired value. Tap and hold for 1/2 second to toggle between normal and CINE mode.
MODE/AVG: Tap and release to se the exposure mode between A (f-number priority), S (speed priority) and dEV (under/over exposure). Tap and hold for 1/2 second to toggle between average (a filled rectangle is shown in the LCD display) and spot (a rectangle with a dot is shown in the LCD display).
MEASURE: Tap to make a measure.
- and +: decrement or increment either ISO, f-number or speed depending on the selected mode.

AVERAGE
When a filled rectangle is shown at the right of the ISO text in the LCD, the exposure is calculated as average of the whole photo.

SPOT
When a rectangle with a center dot is shown at the right of the ISO text in the LCD, the exposure is calculated for the area inside the circle.

MODE A
In f-number priority, buttons - and + set the f-number, and the speed is calculated.

MODE S
In Speed priority, buttons - and + set the speed, and the f-number is calculated.

MODE dEV
In dEV mode, the under/over exposure is calculated (in step of 1/3EV) with respect to the selected speed/f-number.
Buttons - and + changes between pairs of speed/f-number with equal EV.