Re: [RFC 0/4] iio: light: add driver for Broadcom APDS9999

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On Mon, May 11, 2026 at 01:21:12PM +0100, Jonathan Cameron wrote:
> On Mon, 11 May 2026 12:14:42 +0200
> "Jose A. Perez de Azpillaga" <azpijr@gmail.com> wrote:
>
> > This series adds support for the Broadcom APDS9999 ambient light
> > sensor. The APDS9999 is a digital proximity and RGB sensor with
> > ALS capability, featuring I2C interface, programmable gain and
> > integration time.
> >
> > This chip is considered by Broadcom to be the 'successor' to the
> > APDS-9960 which has technically been discontinued.
> >
> > The initial driver implements the ALS/Lux functionality using the
> > green channel, which approximates the human eye spectral response.
>
> That's not normally a good approximation unless it is also has
> very bad colour curves for green!  Better than any other channel
> on it's own but still not good. If we had calibrated sRGB it would be:
>
>  Something like: 0.2126R+0.7152G+0.0722B
> for example.
>
> The lux vs white light and lux vs incandecent light are fairly linear
> so 'maybe' it's good enough but it's a bit of a stretch to describe it
> as an illuminance measure which is what we'd need for it to be measured
> in lux.

the datasheet states: "Approximates human eye response with green
channel" and "Uses optical coating technology to emulate human eye
spectral response."

that's the reason why I chose to use green channel.

> So I'm on the fence at the moment.  We might be better off teaching userspace
> code to do it's own approximation if it has an RGB sensor.

I'll also add IIO_INTENSITY channels for R, G, B in v2 so userspace can
comapute its own weighted lux if it prefers.

--
jose a. p-a