Extract Nature.com – Scientific Reports 7/2017 *

Quality Comparison of potential light sources
for Holographic Projection

T he test from Yuanbo Deng & Daping Chu, published in Scientific Reports 7: 5893 (*), compares the Coherence properties of the following 5 light sources and their effect on the image sharpness and speckle of holographic displays:

DPSS Laser

Diode-pumped
solid-state Laser

LD

Laser Diode

SLED

Superluminscent LED

LED

Light Emitting Diode

mLED

Micro LED

For the test a Superluminescent LED λ = 662 nm from EXALOS was used.

(c) Photos: Yuanbo Deng & Daping Chu. Scientific Reports Volume 7 *

Speckle Contrast (left)

SLEDs achieve a low speckle contrast due to their broad optical spectrum (low temporal coherence).

Image Sharpness (right)

SLEDs achieve a high sharpness due to their high spatial coherence.

Temporal Coherence / Spatial Coherence of 5 light sources.

Temporal Coherence vs. Speckle Contrast

(c) Measurement graph: Yuanbo Deng & Daping Chu. Scientific Reports Volume 7 *

The graph shows the relation between temporal coherence value and speckle contrast value (less is better). The temporal coherence values of the tested light sources are quite different.

The results are proof, that the amount of speckle are mostly influenced by the temporal coherence of the light source.

 

All three LED types were measured in the low value zones, DPSS laser and LD in the high value zone. Speckle contrast and the temporal coherence are in linear relation. SLEDs perform exceptional well and provide precise holographic images with less speckles (see images below).

Spatial Coherence vs. Image Sharpness

(c) Measurement graph: Yuanbo Deng & Daping Chu. Scientific Reports Volume 7 *

The graph shows the relation between spatial coherence value and image sharpness value. The measurements confirm a directly proportional relationship between the spatial coherence value and the image sharpness value.

So the sharpness of the reconstructed images in holographic displays are mostly influenced by the spatial coherence of the used light source.

The spatial coherence values of the LED light sources are in a range from 0.49 to 0.88 – with SLEDs providing the highest values. These values achieve distinguishable holographic reconstructed images with good sharpness (see comparison images on top).

DPSS laser LD SLED LED mLED
Normalized Spatial Coherence 1 0.92 0.76 0.73 0.56
Normalized Temporal Coherence 1 0.81 0.2 0.11 0.045
Speckle Contrast 1 0.76 0.25 0.11 0.055
Image Sharpness Value 1 0.87 0.55 0.22 0.04

Measurement Conclusions *

“Coherence property of a light source can be characterized by its temporal coherence and spatial coherence values, respectively. Light sources such as DPSS laser, LD, LED, sLED and mLED have been characterized and the corresponding holographic images displayed.

Image sharpness and speckle are influenced by both temporal coherence and spatial coherence of the light source in use. It is found that the image sharpness value is linear proportional to spatial coherence value, while the speckle contrast value is linear proportional to the temporal coherence value.

Temporal coherence is decided by the intrinsic spectrum bandwidth of the light source and it can be improved by filtering the spectrum of the light source. On the other hand, spatial coherence is influenced by the size of the light source and the propagation distance in use, it can be improved by changing the size of the utilized light emitting area or the light propagation distance. For example, in an LED based holographic display system, a spatial filter is often applied to reduce the utilized size and increase the spatial coherence of the LED source.

 

Consequently, a light source with high spatial coherence and low temporal coherence is ideal for a holographic display in order to obtain high quality images with good sharpness and minimum speckle. sLEDs and mLEDs, which are not commonly used in holographic displays, are suitable light sources for this purpose.

LEDs with a broad spectrum can also be used to reconstruct holographic images with less speckle, but it has to be spatially filtered for reconstructive sharp images. Otherwise there will be significant reductions in the energy efficiency and brightness of the reconstructed images.”

* Coherence properties of different light sources and their effect on the image sharpness and speckle of holographic displays (2017).

Yuanbo Deng & Daping Chu. Scientific Reports Volume 7, Article number: 5893 (2017) / DOI:10.1038/s41598-017-06215-x
Published with Open Access under Creative Commons Attribution 4.0 International License
External Link: www.nature.com – Scientific Reports

More from our Blog

Highly Efficient RED SLEDs at 625-650 nm

Highly Efficient RED SLEDs at 625-650 nm

Superluminescent light-emitting diodes (SLEDs) are very attractive as compact, highly efficient light sources for various applications in the visible red spectral region. EXALOS investigated the electro-optical performance of SLEDs operating at center wavelengths of 625-650 nm with interesting results.

read more
My Virtual Pet. AR Concept.

My Virtual Pet. AR Concept.

Remember the Tamagochi? Here is the next evolutionary step. With AI just around the corner, virtual living animals will become (augmented) reality. And do we need to start thinking about virtual kids?

read more

EXALOS AG is a privately held Swiss technology company, developing industry-leading Superluminescent Light Emitting Diodes and Swept Laser Sources for the medical imaging, fiber optic gyroscope, test equipment, space, military and sensor industries.

EXALOS offers the widest range of visible SLEDs, including the world’s only true BLUE & GREEN SLEDs, SLED-based Transceivers for Fiber Optic Gyroscope and Current Sensing applications.

You are interested in our VISIIIBLES RGB technology and need more information?

Pin It on Pinterest

Share This