Dan's mugshotDaniel H. Bennett was born in London in 1965 and lived in England until moving to the United States in 2000, settling in Northern Colorado.

His interest in vision and observation of the behavior of light has been life-long. On a trip to France as a young man his hosts called him “Monsieur Tête-en-l’air” (Mr. Head-in-the-air) due to his always watching for halos, rainbows, sun-dogs (parhelia), etc. He was particularly influenced by a book written in 1939 (published in English in 1954, Dover) by Professor M. Minnaert, called The Nature of Light and Colour in the Open Air. A Field Guide to Time-Varying Light Sources was conceived as a modern successor to that book, and adopts a similar approach, while revealing the secrets of modern light sources.

Daniel is married with four children, a motorcycle and two accordions.

Buy the book!

The Field Guide is available from Amazon and other outlets.

The book's front cover

ISBN-13: 978-069-236090-3

ISBN-10: 0-692-36090-5

LCCN: 2015900348

Purchase the book on Amazon!

The Blog

Here's my blog, where I'll write articles of interest, things I've learned, tutorials, suggestions for lights to go look at, musings, etc.

Thursday, 02 June 2016 19:02

Exhibiting artist at a coffee shop

Written by

I'm excited to be the exhibiting artist this month at the Land Of A Thousand Hills coffee shop in Roswell, GA!

These photos are all from my time-varying lights project, and were selected from my online gallery: (where you can order prints if you don't happen to be in Roswell, GA... laughing)


Friday, 11 March 2016 11:46

Arcturus - twinkling star

Written by

Two nights ago I took a photo of the star Arcturus, using my Canon 70D camera with a 200mm zoom. The exposure time was 1.0 second, during which time I hand-held the camera and moved it during the shot. Here's the result.

The trace shows a massive number of variations (over 800!) of brightness and color. This is scintillation, better known as twinkling. Our eyes aren't able to see all these variations because (a) they're too dim, so we don't see the colors, and (b) they're too fast, so we see only the largest and slowest variations in brightness as twinkles.

The cause of these variations is atmospheric - the star itself is NOT twinkling! If you took a photo like this from the International Space Station, it would be a constantly bright, slightly orange line with no variations in color or brightness.

This type of photography is really easy... go try it!

A 1-second trace of Arcturus' scintillations

Tuesday, 02 February 2016 19:40

Featured on Light Painting Photography website

Written by

I'm very excited to be the current featured artist over at Light Painting Photography, Jason Page's great nexus for all things light painting. Jason worked with me to create a great interview, sprinkled with many of my photos and links back to here. Thanks, Jason! It's humbling to see my rather unique photography being showcased to such a creative crowd of people, and I hope that it stimulates some new ideas and techniques. I'm looking forward to seeing where this new endeavor takes me.

Saturday, 12 December 2015 17:59

A city from the air

Written by

In case you weren't already convinced that most street lights are time-varying, here's proof. This is a shot I took of a small US city (I don't know which) from an airplane at night. I swept the camera while taking the shot, using a 1/10 second exposure time. As you can see, there are many lights and groups of lights which show strong periodic traces. Most of these lights are pressurized sodium street lights, which flash at 120 times per second (in the US).

I had to adjust the exposure and tone curve in Lightroom to compensate for the fact that a city at night isn't very bright as far as a camera's concerned; and when you sweep the camera, it's even dimmer.

Swept image of a city from the air

Friday, 27 November 2015 21:46

Christmas lights

Written by

Christmas is coming... and LED Christmas lights are appearing. Almost without exception, these LEDs flash at 60 Hz (in the US, 50 Hz in Europe). The result: move the camera and you'll see the dashed-line trace of each LED. Here's an example...

 LED lights on trees at Christmas

Saturday, 07 November 2015 20:23

Mirroring an image

Written by

Inspired by something I saw online, I decided to try reflecting an image in horizontal and vertical axes using Photoshop. Here's the result:

Reflected image of pressurized sodium lights in Albuquerque, NM

The original, non reflected image can be seen here.

Sunday, 01 November 2015 21:14

Cellphone camera for checking out light sources

Written by

I've made a cool discovery: cellphone cameras (and probably most cameras with a live display (sometimes called "live view") can be used to observe time-varying light sources.

Simply open the camera app and point the camera at the light source of interest, then rock the camera rapidly up and down so that the light source paints a trail on the screen. In most cases, you'll be able to see the trail breaking up into dashes or repeated images, showing that the light source is time-varying. It's not as good as eyeball- or mirror-jiggling (see my "How To See" page), but as a quick way to see if a light has time-varying behavior, it can be useful.

I've tested it so far on three cellphones: LG G4, Apple iPhone 6+ and Motorola Droid Mini. It helps to let the camera focus on the light source first before rocking the camera up and down.

I've just moved to the LG G4 phone, and was excited about its camera's manual mode. Tonight I had a chance to try it out. I photographed this restaurant, which has a line of white LEDs around its windows:

 Restaurant with white LED decoration around the windows

By using the camera's manual settings, I captured this image:

 Vertically swept photo of the restaurant

The time-varying behavior of these LEDs is very easy to see with the naked eye, and may well be one of the easiest subjects for vision jiggling.

To get this image, I selected the following settings:

  • Manual focus (so that it would not be trying to focus while taking the shot)
  • ISO 500 (to underexpose the image so that I get the lights, not the whole scene)
  • 1/8 second (to capture for long enough to give me plenty of dashes in the traces)

The primary weakness with the G4's camera is that there's an appreciable delay between pressing the button and the shot being taken, and since you have to keep the camera moving it's unpredictable what you'll capture. The solution is to take several shots until you get what you want. Aside from that, though, and the lack of optical zoom (which isn't too much of a problem for this type of photography), the G4 makes a great pocket tool for the photographer of time-varying light sources. Highly recommended.




Sunday, 25 October 2015 21:40

Follow me on Twitter!

Written by

I'm ramping up on Twitter. Feel free to follow me: @TimeVaryLights . Enjoy!

Sunday, 26 April 2015 16:41

Wonderful three-phase arrangement

Written by

This is a slightly cropped region of a photograph I took at Indianapolis airport in March 2015. It shows a long building with pressurized sodium lights along the front. The cool thing to note are the regions where the traces show strong diagonal patterns to the light and dark dashes. This is due to the fact that the electrical contractor who wired up the lights was fairly rigorous in following a 1 - 2 - 3 - 1 - 2 - 3 sequence when selecting which phase of the three-phase electrical supply each light should be connected to. This is a sensible approach as it balances the load nicely. And, for those who know how to see it, it makes a cool pattern!

Lights on a building at Indianapolis airport
Unusually regular three-phase patterns
Thursday, 23 April 2015 08:31

Price drop

Written by

I've decided to drop one of the distribution channels that my book could - maybe - be sold through. That has allowed me to drop the list price from $32.99 to $25.99. This price change takes effect immediately on Createspace's page, but may take a day or two to be reflected on Amazon's page.

Wednesday, 04 March 2015 19:06

Featured on Atmospheric Optics website!

Written by

Exciting news regarding my book: Today, it's being featured on the Atmospheric Optics Picture Of The Day website! Although most of the book isn't concerned with atmospheric optics per se, the section on twinkling stars certainly qualifies; and the website's charter is pretty broad, so it fits right in.

The permanent link to the page is

I asked the guy who runs the site (in which I had a photo published a while ago) if he would feature the book, and he's done an excellent job! He obviously had a lot of fun building that collage.

His audience is my audience, so I'm hoping this will help to get the book some targeted publicity.


Saturday, 07 February 2015 15:23

Welding and laser cutting at Walker Mowers

Written by

A mower from WalkerI had the chance to take a tour of Walker Manufacturing (home of the high-end commercial mowers) this week. This was my first chance to see electric welding equipment in action, so I took the opportunity to eyeball-jiggle at the bright welding spot. Sure enough, it's time-varying, but it turns out it's varying in two ways:

  1. It fluctuates in brightness a lot, as you would expect when welding is taking place.
  2. But it also flashes on and off rapidly, probably at 60Hz or 120Hz, which shows that its power is modulated by the A/C cycle.

Obviously, one shouldn't do this for too long; but actually, eyeball jiggling has the effect of reducing the amount of energy reaching any one spot on your retina. As long as you don't stare at it like a deer in the headlights, you'll be fine.

I also had the chance to see a heavy-duty laser cutter in action. It's cool! It takes large sheets of steel, which can be up to half an inch thick and weighing up to 2000 lbs, and it cuts shapes out of them as if they were butter! I was able to observe the bright spot right where the cutter is melting the steel, and sure enough, there was a regular variation in brightness. It seemed quite fast, maybe 200 or 300Hz, though it would need a photograph to allow one to measure the frequency. I'm pretty sure I wasn't seeing the laser itself, but variations in the temperature of the melting spot, maybe. The variations were fairly subtle.

It's always worth jigglin' at stuff - you never know what you might see!

Wednesday, 28 January 2015 16:17

A stack of books

Written by

20 copies of the book arrived today - the first time I've seen the final product in physical form. This is a wonderful moment for me... the tangible fruit of a lot of labor!

Site design

This site is powered by Joomla! CMS. The template is "Cast" from Joomlage, modified here and there. The gallery is powered by RokGallery. All design by Dan Bennett.


All content on this website, including images, videos, text and material from "A Field Guide To Time-Varying Light Sources" is © 2023 Daniel H. Bennett / / A Bear Peering Round A Rock, and may not be reproduced without permission. All rights reserved to the extent of applicable law. Exceptions: third-party images, which are credited as applicable, third-party embedded videos and public domain images.