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OPT4001-Q1: Calculation method of Lux from measured value on register and the criteria of change measurement range by auto range mode

Kazuki Kuramochi
Expert 8810 points
Part Number: OPT4001-Q1
Other Parts Discussed in Thread: OPT4001

Hello Expert,

Would you answer following question?

  1. Could you explain how to calculate lux from measured result on register using example?
    Datasheet has explanation but it is using some digital calculation as like as << and 400E-6.
    It make's difficulty of understand.
    Therefore, please explain the calculation using example.
  2. Could you explain criteria of change measurement range at auto range mode?
    Datasheet's explanation using unclear words as like as toward the low/upper side, decreasing one or two, and so on...
    Therefore, we cannot understand when range update happen.
    So I need more clear criteria and explanation to understand the auto range behavior.
  3. How long will it take to restart the measurement using update range in continuous mode?
    Datasheet's explanation seems that it restart measurement immediately if it detect saturation of measurement range.
    However, I cannot how long it will take to restart measurement.
    We need this value to make decision of their register read cycle in continuous mode.

Thank you and best regards,
Kazuki Kuramochi 

  • 0
    TI__Genius 9675 points

    Hi Kuramochi-san,

    1. Let's take a look at this screenshot from the GUI and calculate 1000 lux from the information provided. The simplest way to calculate the lux would be the equations 4 and 5.

    • ADC_CODES = Mantissa * (2^Exponent)
    • lux = ADC_CODES * 400e-6

    Let me mention that the coefficient (400e-6) is different dependent on package type. In the GUI screenshots I was using the OPT4001 DTS package (commercial part) the coefficient for this package is 437e-6. So I will use 437e-6 in my example calculation.

    GUI Screenshot

    The GUI screenshot shows the Mantissa value of 0x8CB00 and Exponent value of 0x02.

    Converted from hex to decimal -> Mantissa = 576256 and Exponent Value = 2. Plug these values into the equations from above.

    ADC_CODES = Mantissa * 2^E = 576256 * 2^2 = 2305024

    lux = ADC_CODES * 437e-6 =  2305024 * .000437 = 1008

  • 0
    TI__Genius 9675 points

    2. The change is based on current lighting conditions and the previous measurement. The range will either decrease by two steps or increase by one step as it takes a measurement.

    3. This is determined by the conversion time setting. If there is a large transient response in light then there can be a delay in measurement, due to the first measurement being aborted then retaken.

  • 0 in reply to Jalen Tate89
    TI__Expert 8810 points

    Hello Jalen,

    2. Could you provide more explanation when range will decrease by two step or increase by one step?
        Our expected answer is "current measurement result is greater/smaller than twice/half of previous measurement then range will be changed.

    3. Could you provide worst case delay? (e.g. double of current conversion time?)


    Best regards,
    Kazuki Kuramochi

  • +1 in reply to Kazuki Kuramochi
    TI__Genius 9675 points

    Kuramochi-san,

    2. If the current measurement is less than 25% of the previous measurement then it will decrease one step. If the current measurement is greater than 75% of the previous measurement then it will increase by one step.

    3. I am checking with our design team on this and will let you know their feedback.

    Thanks,

    Jalen