gilshultz:
I use the barrel jack or Vin input all the time,
Good for you!
gilshultz:
I power it with 7.5 Volts to 8.5 volts however I do not use the board to power anything external
OK, so you have bought out of this discussion as he is using something external - a LCD shield. So, let's get back to the situation in hand.
Gabr21:
Indeed, when powered via USB I measure a voltage of 4.9 volts at the 5v pin. When powered via the jack (close to 8volts) I measure around 4.15 volts at the 5v pin (is such a voltage drop to be expected?).
Yes and no. It is a little surprising that you are seeing quite as much drop.
In regard to your original questions, the 200 mA for the ATmega chip is a parameter of the chip itself given an unrestricted 5 V supply. It means you must not draw more than that current from the I/O pins of the chip. It has nothing to do however, with the voltage regulator on an Arduino UNO. The ATmega chip itself draws some 40 or 50 mA and the mega16U2 USB interface chip, somewhat less.
The LCD draws a milliamp or so with an extra half milliamp due to that wiring blunder I mentioned. The backlight usually draws about 25 mA though some odd displays draw more. There is no other significant current draw on the keypad shield.
And you mention that your sensor draws 12 mA. Arguably the regulator should be sufficient under these limited circumstances but in general, it is a bad idea to use it.
Gabr21:
I don't understand however how this voltage difference is translated into a difference in the intensity of the display. Am I providing less current because of the regulator?
OK, there is (always) some confusion about the LCD display. Its brightness relates only to the backlight LEDs and you do not (generally) adjust that, There will be a little drop in intensity between 4.9 and 4.15 V as the LED is operating at about 3 V so you are looking at a variation from 1.9 to 1.15 V across the resistor and the LED current is accordingly different.
What you are actually seeing is variation in contrast and this is quite sensitive to voltage (which is why the potentiometer is required in the first place). Half a Volt corresponds to almost the whole contrast adjustment rage.
In summary - simply don't use the on-board regulator; find a nice regulated 5 V supply of suitable scale.