<p>It might not be obvious in your example, but reduced signal is apparent when you analyze timelines of LiDAR.  For instance, see the example below from the NASA site on <a href="https://earth.gsfc.nasa.gov/meso/research/atmospheric-profiling" rel="nofollow noreferrer">atmospheric profiling</a>, or <a href="https://mde.maryland.gov/programs/air/AirQualityMonitoring/Pages/Lidar.aspx" rel="nofollow noreferrer">this site</a> which has a <a href="https://mde.maryland.gov/programs/air/AirQualityMonitoring/PublishingImages/Lidar_particlesmixdown.png" rel="nofollow noreferrer">great example</a>.  There is no signal above thick cloud or smoke layers, indicated by complete loss of color above the cloud.  Depending on the wavelength you are looking at, you may see more or less of this signal loss.</p>
<p><a href="https://i.sstatic.net/fV3pV16t.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/fV3pV16t.jpg" alt="NASA LIDAR" /></a></p>

CC BY-SA 4.0
2024-12-10T03:19:38.317Z
2024-12-10T03:19:38.317Z
f.thorpe
<p>Let's say LIDAR is used for atmospheric vertical sounding. If backscattering takes place  some amount of the outgoing radiation is scattered back to the LIDAR. But if this is the case, the vertical power of the beam must get smaller and smaller. In the LIDAR equation I cannot identify this : The received power in the detector is</p>
<p><span class="math-container">$P_r \propto \beta(R) \cdot exp(-2 \int_0^R \alpha(r) dr)$</span></p>
<p>where <span class="math-container">$\beta$</span> is the backscattering coefficient and <span class="math-container">$\alpha$</span> the absorption coefficient. If there is no absorption, how can the beam not get weaker? I would expect, that any scattering also goes into the term with the optical thickness.</p>
<p>How can it be, that in the image we can &quot;see&quot; the upper object, with big intensity, although the beam must be weakened by the lower dust and clouds.</p>
<p><a href="https://i.sstatic.net/jtOhgwoF.png" rel="noreferrer"><img src="https://i.sstatic.net/jtOhgwoF.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2024-12-09T20:01:09.323Z
You notice the exponential decay in your formula?
planetmaker
<meteorology><atmospheric-radiation><radiosounding><lidar>
How does LIDAR work? Why is scattering not weakening the beam?
MichaelW
# How does LIDAR work? Why is scattering not weakening the beam?

(Asked by: username_0 on 2024-12-09)

Let's say LIDAR is used for atmospheric vertical sounding. If backscattering takes place some amount of the outgoing radiation is scattered back to the LIDAR. But if this is the case, the vertical power of the beam must get smaller and smaller. In the LIDAR equation I cannot identify this : The received power in the detector is

$P\_r \propto \beta(R) \cdot exp(-2 \int\_0^R \alpha(r) dr)$

where $\beta$ is the backscattering coefficient and $\alpha$ the absorption coefficient. If there is no absorption, how can the beam not get weaker? I would expect, that any scattering also goes into the term with the optical thickness.

How can it be, that in the image we can "see" the upper object, with big intensity, although the beam must be weakened by the lower dust and clouds.

[![enter image description here](https://i.sstatic.net/jtOhgwoF.png)](https://i.sstatic.net/jtOhgwoF.png)

**Tags:** meteorology, atmospheric-radiation, radiosounding, lidar

> **Comments:**
> **username_1**: You notice the exponential decay in your formula?  

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## 1 answers

### Answer by username_2 on 2024-12-10. Score: 3

It might not be obvious in your example, but reduced signal is apparent when you analyze timelines of LiDAR. For instance, see the example below from the NASA site on [atmospheric profiling](https://earth.gsfc.nasa.gov/meso/research/atmospheric-profiling), or [this site](https://mde.maryland.gov/programs/air/AirQualityMonitoring/Pages/Lidar.aspx) which has a [great example](https://mde.maryland.gov/programs/air/AirQualityMonitoring/PublishingImages/Lidar_particlesmixdown.png). There is no signal above thick cloud or smoke layers, indicated by complete loss of color above the cloud. Depending on the wavelength you are looking at, you may see more or less of this signal loss.

[![NASA LIDAR](https://i.sstatic.net/fV3pV16t.jpg)](https://i.sstatic.net/fV3pV16t.jpg)


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