A light meter is only as good as it is accurate. I'm happy to say that this Lumu Power light/color meter is very accurate. I have a Sekonic L758DR light meter and I took simultaneous readings both with that and with this Lumu Power on my iPhone 7. I changed the settings on the Lumu Power to display exposure in full stops plus 1/10ths, which is what my Sekonic displays. The Lumu was always over the Sekonic reading, typically 2-3 tenths of a stop. The biggest discrepancy was one reading where the Lumu was over 4 tenths of a stop. I tried changing the calibration settings of the Lumu to +0.2 EV, and then the Lumu readings matched the Sekonic much more closely.If you're really super serious about accuracy, you should perform a calibration for your light meter with each camera sensor, using a controlled light source with an 18% grey card, taking exposures with the camera and dialing it in until you get a 50% luminance reading on the image of the grey card. Every light meter should be calibrated for best results, including the Lumu Power. If you just want ""meh, I'm sure it's close enough"" accuracy, then this Lumu Power will give you good results out of the box.The Lumu Power can be plugged in two ways - with the dome side facing forwards or with the flat side facing forwards. The dome side is used by the Lumu app for the PHOTO AMBIENT, PHOTO FLASH and CINE/VIDEO modes to measure the amount of light falling on the dome. The flat side is used for ILLUMINANCE, COLOR TEMPERATURE, and CHROMATICITY modes. The app will take readings regardless of which side is facing forwards - so if you're getting unexpected readings, make sure you've got the correct side facing forwards.There's very little documentation included in the packing, but the Lumu website has lots of info and an in-depth instruction manual PDF.The best thing about this Lumu Power, in my opinion, is that is has a color temperature meter in addition to being a light meter. My Sekonic doesn't have that. Sekonic color meters are very expensive ($1500+). Even ""off-brand"" color meters can run almost $1000. So, to have that capability in such a tiny phone accessory is pretty amazing. I don't have a color meter to compare the Lumu Power with, but I've taken readings in several different situations for video shoots (daylight, shade, tungsten, fluorescent), and it always seems to give the reading you'd expect, so I trust it to give me a reading that gives me a good in-camera image. Keep in mind that the color meter takes a reading only from the flat side of the meter. I've forgotten a couple of times and taken a reading from the dome side when really it was reading the back (flat) side.Because this is an iPhone accessory with a dedicated app, firmware updates are easy. In fact, just today as I write this review there was a firmware update to improve accuracy of the color meter. It only took a few seconds to update (and it was optional, so you won't be stuck doing updates in the middle of a photo shoot when time is of the essence). When I first started using this light meter, I had occasional difficulties getting the Lumu app to recognize the meter, but firmware/software updates have since fixed that and now the app pairs perfectly every time I plug the light meter in.The only downside I can see of the Lumu Power is that it's only good for as long as Apple keeps the Lightning port on its iPhones. As of the date of this review there are rumors that Apple might switch to the USB-C standard for its next iPhones, so if that does happen and you wanted to upgrade your phone then you wouldn't be able to use this light meter (without some sort of adapter that Apple will inevitably release).One minor thing I'm not crazy about is the included case. It's a small fake-leather pouch with a snap button that just barely fits the light meter. It doesn't offer a whole lot of protection. I use a small plastic hard case (in addition to the included pouch) to keep the Lumu Power protected in my camera bag.The fact that the Lumu Power has a light meter, flash meter, and color temperature meter in a tiny device that plugs into your phone is truly amazing to me and I think it's a very valuable piece of equipment for any serious photographer/videographer. I'm using this professionally and it is serving me very well.