VL53L4CX which distance mode should I choose?

[Andrew12]

There are three modes: LONG, MEDIUM, and SHORT. The names suggest that LONG allows measuring the greatest distances. In document UM2923 (section 5.2), there is information that the maximum distance can be achieved with the MEDIUM setting. What setting should I ultimately choose to measure the greatest distances?

[John E KVAM · Accepted solution]

This is a little confusing, but here you go.

When we invented the chip, we had three modes. 

In 'short' we made the pulse repetition rate short, so in a giving amount of time we could send a lot of pulses, and get a lot of photon times. To range a bit farther we had Medium. Longer period between pulses, but you could range farther.

And of course 'Long' has the longest period between pulses - but you could get the longest range.

Now the tricky bit...

We invented 'extended range'. 

During normal ranges we have a problem with 'Radar Aliasing' or wrap-around. This is when a target is beyond the max range, and the returning photons are associated with the wrong out-going pulse. It makes targets at 4.3 meters look like they are at 0.3 meters (when the wrap point is at 4M.) To prevent this we use two different pulse repetition intervals. (Call these A and B)

If range A is the same as range B, then we no issues. If they are different we know we have a wrap-around.

Then some clever folks realized that we could use the difference in range A and B results to not only detect the wrap issue, but to figure out where the target is. 

So instead of a max 4M range, we could go out to 8M is some interesting corner cases - like when looking at a highly reflective projector screen.

But we notices that the best combination of timings for extended mode was "Medium'.

So, if you have extended mode turned on, use Medium. 

If you want more signal, but are willing to give up distance to get it, use Short.

If you don't want extended mode, but need 4M, use 'Long'.

But for extend mode, Medium works best.

- john

[Andrew12]

Thank you, now I understand. Since we're talking about an extended range - I have an additional question: Can the power or any other parameter be increased to detect dark objects at a greater distance?

[John E KVAM]

Short answer - NO!

The 940nm light can enter ones eye and hit the retna. It's potentially dangerous.

As such the FDA has very strict rules regarding how powerful that light can be. 

Unless you can guarentee the user can never see into the laser, you have to live with the power we output.

Sorry about that. But safety has to come first. 

- john

[Andrew12]

I would like to improve the detection of objects with low light reflectance. I was wondering if the power is adjustable, and perhaps the default value is not the maximum available. If the power cannot be increased for safety reasons - ok (safety first). Do you have any other suggestions on what can be done to better detect dark objects?

[John E KVAM]

1) Short has more radar 'pings' per second than does Medium. And Medium has more than Long. So choose the shortest one that covers your required distance.

2) Longer integration times can 'see' more as there is more time to gather signal.

Other than that you have to brighten up the target. Adding a tiny spot of retro-reflective material can do it. 

Past that you have no choice other than switch to the VL53L1CB (same software, hardware is footprint compatible.)

The L1 is taller, but has a Lens that gathers more light. So it 'sees' weak targets better. 

- john

[Andrew12]

Indeed, the L1 has a larger lens, and I was contemplating using it. However, the documentation does not clearly specify the maximum range, whereas for the L4, it is explicitly stated as 'up to 6 meters.' That's why I chose L4. Now, I will conduct tests with the L1. Thank you for help.

[Andrew12]

Now I'm using the VL53L1CB and it really works better. Do your range mode selection tips also apply to this sensor? I ask because I'm now monitoring an additional parameter: RecommendedDistanceMode, and sometimes it suggests a LONG setting.

[John E KVAM]

It's a tradeoff between the number of ranges you get per millisecond and the distance you need. If you want to only range a short distance, we could put the flashes of light closer together. More pings means more accuracy. 

But if you want to go past 4M, then you need a combination of more pings, and you need the result of both phases of the range. Medium is the best tradeoff. 

If you wanted to limit yourself to 4M, then Long makes sense.

(I think we've killed this topic. If you have more questions, please open a new topic. It's helps others search.)

- john