You shouldn't really be getting the voltage from the TextBox. You must have had a numerical value in the first place in order to display it in the voltage TextBox. It's that numerical value that you should be multiplying by your conversion factor. Ideally you would then convert the result to a String before displaying it:
vb.net Code:
TextBoxVoltage.Text = voltage.ToString()
TextBoxLux.Text = (voltage * 1333).ToString()
The Text property of a TextBox is type String so, ideally, you should be assigning a String to it. With Option Strict Off the system will make the conversion implicitly. The sooner you turn Option Strict On and force yourself to make conversions explicit the better because you'll learn better habits and your code will perform better and be less brittle.
That said, your conversion isn't going to be a simple multiplication by a constant like that. That only applies to that specific piece of equipment. Think about it. If lux is a measure of luminous emittance then it can't be a linear relationship to voltage. Different light sources are going to emit different levels of illumination for the same voltage. That page says:
Coupled to a 10Kohm resistor, and given the specification of the BPW34
You would need to determine the conversion factor that applies to your own combination of resistor and photodiode or whatever equipment you're using.
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