I want to make a USB LED Lamp

USB Lamp Circuit

Here is a simple USB powered lamp that can be used to light your desktop during power failures. The circuit operates from the 5 Volt available from the USB port.The 5V from the USB port is passed through current limiting resistor R2 and transistor Q1. The base of transistor Q1 is grounded via R1 which provides a constant bias voltage for Q1 together with D2.The diode D1 prevents the reverse flow of current from battery.C1 is used as a noise filter.Two white LED’s are used here for the lamp, you can also use a 2 V torch bulb instead of LED’s. LED D3 indicates connection with USB port.

USB Lamp Circuit Diagram with Parts List.
USB port is only able to provide up to 100 mA current.So don’t overload the circuit with more no of LED’s.
Before wiring the circuit confirm the positive and ground leads of USB by a multimeter.
Switch S1 can be used to turn on the lamp.
Then Currently I have a project that consist in the following:

We have 2 USB Led lamps. (e.g 13 leds, 5V, ~200mA)
To these lamps, we would like to control the intensity of the light, through a Raspberry PI.

My main question would be what would be the simplest and cheapest way to achieve this?

So far from what I have seen in internet a digital potentiometer would work on this, maybe using a DS1669 or a AD5204 such as http://www.kynix.com/Detail/12166/AD5204BRUZ100-R7.html.

Sorry if this question is too stupid, but I’m out of my field here, and I would like some suggestions as how to go about this. So if anyone is willing to put me in the right direction.