What Lighting Do I Need for Slow Motion?

Slow motion footage in a video can add drama and style, but to make it work you need to have the right kind of lighting. This is a key factor in creating correctly exposed, flicker-free slow motion videos.

Lighting is critical in video for translating images into something the human eye will perceive as acceptably real. Even though we do not see real things in slow motion, we still have a visual sense of how slow motion can heighten the sense of reality of what we are seeing.

The correct lighting equipment is essential if you are going to deliver the highest quality slow motion video. Here, we look at what lighting technology works best for slow motion, including:

• HMI
• LED and
• Plasma lighting

Variables in Slow Motion Filming

There are certain key variables you must consider if you are shooting a slow motion video:

• Frame rates
• Shutter speed
• Lighting

You cannot create impactful and fluid slow motion if you do not control all of the variables involved.

This fluidity is a major element, because without it, your slow motion video will look jumpy and unprofessional.

If you do not address the frame rate and simply slow down a 25fps recording, there will not be a smooth transition between each frame.

Therefore, you should record at a higher frame rate (i.e. 150fps) then play back at a lower one (25fps), giving you 6 times slower than real time speed (in this example).

When working out the optimal frame rate, you must look at the speed necessary to capture the action you are filming, and what length of video you will need to then play the action back later at normal speed.

This comes down to the visual aesthetics you want to capture. For example, 60fps may be right to achieve your slow motion effect, but in certain instances you may require a higher frame rate.

Adding the extra frames in the lower frame rate for playback creates a smooth motion.

Shutter speed is another important variable. Controlling shutter speed enables you to control the amount of blur. The longer the shutter stays open, the blurrier the motion.

To get a natural-looking amount of motion blur, the general rule is to set your shutter speed at double your frame rate.

However, a too-fast shutter speed will make your slow motion footage appear choppy, so this is about getting the speed just right. You should, therefore, also consider how much movement there will be in the subject you are filming.

What using faster shutter speeds and frame rates also means is that your camera will have less time to capture light for each frame you shoot. Consequently, you need more powerful sources of light if you are going to expose your shots properly.

Without the right amount of light, you will end up with slow motion footage that is underexposed. Without the right type of light, you may end up with slow motion footage that flickers.

This is why lighting is a critical aspect of shooting in slow motion.

Why Flicker is a Lighting Issue

The flicker in tungsten lighting sources is due to the frequency of AC current used in the mains supply to that lighting fixture. In the UK, the frequency of the mains supply is 50Hz, whilst in the USA, it’s 60Hz. Hertz refers to the number of complete AC sine waves per second.

HMI sources inherit the frequency from the supply ballast, usually selectable between 50Hz, 60Hz, and either “flicker free” (usually 100Hz) or in specialist ballast such as the ones that we supply as standard with our ARRI M Series lighting, a special (but noisy!) 1000Hz frequency is selectable, for ultra slow motion images captured with our Vision Research Phantom VEO4K @ 1000fps in 4K.

Most light sources used in filming do not emit a continuous luminous flux, but will have an intensity that varies depending on the voltage of the supply.

As mains voltage changes its polarity per second, so light intensity pulses in time with this mains frequency.

Usually, this effect is invisible to the naked human eye but when filming lighting, you are using set frame rates that, when mismatched with the lighting frequency, can cause the image to flicker or roll. In this way, the frame rate of the camera’s sensor works in a very similar way to the frequency of the AC mains supply. 25p (25 frames per second) has a frequency of 25Hz. When using a 1/50^th or 180 degree shutter on a 25p sensor, the frequency now becomes 50Hz.

Generally speaking, if the frame rate fits in whole number multiples of the lighting frequency, no flicker occurs. But when the frame rate mismatches the frequency, for example shooting 25p in America where the mains supply is 60Hz, flicker will occur. When shooting at 1000fps, a lighting source of 50Hz is far too slow to keep up with the frame rate, so again, in this case the slow 50Hz frequency is visible as flicker at these ultra-high frame rates.

To solve the problem, either adjust the frame rate, shutter speed, or lighting frequency to match.

The choice of light source is critical in controlling the flicker factor. The higher the frequency, the better the lighting will be for slow motion.

Flicker Free Lighting

If you are shooting outdoors using the sun and reflectors as your light sources, you will not have to deal with flicker.

But most electrically-generated sources of light will flicker to some degree, as described above.

Controlling this degree of flicker is really about influencing the perception of the flicker for the viewer.

Some artificial light sources are better than others at providing flicker-free light.

When shooting at a normal, 25fps frame rate, for example, you could shoot with a tungsten bulb, or even a household fluorescent light, and not encounter flicker issues. But once you start varying the frame rate and shutter speed, flicker is much more likely to become an issue.

Different forms of lighting have different flicker factors:

HMI Lamps

HMI lamps can operate on flicker-free electronic ballasts. HMI stands for hydrargyrum medium-arc iodide. These lamps use an arc lamp instead of an incandescent bulb.

The electronic ballast is a device that produces an ignition pulse and regulates the arc. This transforms the sine wave into a square wave, enabling the light to stay constant for longer after zero-crossing (the point where there is no voltage present).

A flicker-free HMI lamp will typically have a low frequency of between 75 and 100Hz. However, over the bulb’s lifetime, the gap between electrodes increases, with the arc extinguishing for longer periods during zero-crossing. This can increase the flicker-factor to above 3%, which indicates that you need to replace the bulb.

LED Lights

Because LEDs inherently react very rapidly to voltage changes, they are prone to flicker, especially if you dim them.

To deal with this, professional quality LED lighting fixtures regulate the frequency of each individual LED on the panel, adding dramatically to the quality of the light (as well as the price tag)!

Because of this system, most LED fixtures are nowadays are regulated to much higher frequencies than we can capture, even with specialist cameras.

New large dot LED technology is rated in excess of 10,000Hz! This makes them an ideal match to high frame rate filming.

Tungsten Lamps

As explained above, tungsten lamps gain their frequency emission from the AC mains supply.

If you dim tungsten lamps, you are adjusting the current it receives, and thus affect the frequency and flicker.

Fluorescent Lamps

You can operate fluorescent lamps on either electronic or magnetic ballasts. This reduces flicker, but both fluorescent lamps and energy-saving bulbs have flicker factors that are generally higher than alternative lighting methods.

With magnetic ballast (such as those common in building strip lights), fluorescent lamps have a flicker factor between 30% and 60%. With electronic ballasts (such as those found in professional fluorescent fixtures such as Kino Flo Divas / 4x4s etc.), you can reduce this to between 0% and 12% (with Kino having a 0% flicker factor)

Where you have to shoot with non-flicker free light sources, you will have to rely on adjusting the frame rate and/or shutter angle to take into account the wild flicker caused by the magnetic ballasts.

There are, however, an increasing number of advanced options for flicker free lighting now available, giving filmmakers more flexibility when it comes to shooting in fluid slow motion.

High Speed Ballasts for HMI Lighting

Although electronic ballast can prevent flicker in HMI lighting, these lamps can also be prone to arc wander. This is where a plasmatic hot spot moves within the bulb, causing a shifting movement in the light output.

This can then result in a shimmering effect with a rapid colour shift.

This means that you cannot completely guarantee flicker-free lighting using HMI lamps with normal electronic ballast.

One solution is the use of high frequency ballasts for HMI lamps. These can reduce significantly flicker, or even eliminate it altogether.

These high speed ballasts of between 300Hz and 1000Hz make it possible to achieve flicker free images of a high quality, at frame rates as high as 1000fps.

M Series and MAX Technology

The ARRI M Series of daylight HMI lamps uses patented MAX technology to provided cutting-edge lighting.

MAX Technology supplies a unique reflector design for this lighting, creating extremely bright, open-face lighting units.

The five HMI lamp heads in the M Series offer a range of even wattage options, from 800W up to 18,000W.

Because they have done away with the need for heavy lenses, these lights combine excellent handling with a high consistency of lighting. This makes them ideal for shooting the high frame rates necessary for good slow motion footage.

Advances in LED Lighting

As we have explained, achieving flicker-free lighting for slow motion using LED lighting is dependent on ensuring the stability of the power supply via a high quality, regulated LED fixture.

Technological advances have now produced LED lights that are setting new standards in the power and portability of LED lighting

• The Litepanels Gemini 2×1 and 1×1 fixtures provide and incredible even and soft light, perfect for slow motion fill applications. These lightweight, battery operated LED fixtures are modular and can be fitted together to create a 4×1 or 4×2 array.

• Stella Pro Lighting also manufactures advanced LED lighting, including its Stella Pro 10000c ultra-powerful LED lamp. This corded light produces cinematic-quality lighting on a par with HMI lamps, and has a consistent, regulated and reliable output that makes it perfect for slow motion filming. It generates 10,000 lumens supported by sophisticated flicker-free firmware. To maintain constant output at its considerable power levels, the Stella Pro 10000c has an integrated variable speed fan for when the light generated exceeds 5,000 lumens.

• The ARRI L Series incorporates a smooth light field and continuous focusability into its LED technology. This gives users complete control over the colour and intensity of LED lighting, making it highly reliable for the high numbers of frames per second essential for professional-grade slow motion shooting.

The Wireless Option for LED Lighting

Another advanced solution for LED lighting in slow motion video is to go wireless.

Astera provides sophisticated, battery-powered lights. These remote-controlled lights are highly portable and designed specifically for shooting a broad range of videos.

The Astera range of wireless LED lights is flicker free. It uses scrambled pulse width modulation (S-PWM) to achieve this.

The duty cycle of an LED refers to the percentage of time the light is on. Normal pulse width modulation is a means of dimming LEDs by rapidly turning them on and off. This is pulsing. Visually, this appears as a steady, dimmed light.

Scrambled pulse width modulations represents a further refinement of this process. It scrambles the pulses of each LED channel into sub-pulses. This allows the channels to co-ordinate to reduce the periods when they will all be off.

This can reduce the flicker effect in high level recordings.

Plasma Lighting

Advances in both HMI and LED lighting technology have made them versatile and adaptable for slow motion filming.

But there is an alternative to both, and this is plasma lighting.

There are no electrodes in a plasma light. Instead, radio frequencies (RF) transmit energy into a quartz bulb containing a mixture of noble gases and metal halides. There is an electrical field in the centre of the bulb, which ionises the gases. They heat up, evaporating the metal halide materials to create a bright light. Since this energised ball of glowing gas is detatched from the mains / ballast frequency, it is completely and truly flicker free.

High output 2500W plasma lights combine various qualities of other lighting forms. They offer the daylight balance and high output of HMIs plus the reliability, long life and energy efficiency of LEDs.

The Hive Plasma 1000 provides flicker-free lighting from a high-speed light source.

It operates at speeds well in excess of the most rapid LED lighting, offering cycles of 450 million times per second.

This far outstrips any currently available frame rates. You can only detect flicker from a plasma light at the equivalent of 225 million frames per second.

Realistically, for practical purposes, plasma lighting at this level is flicker-free, offering a leading edge solution for high quality slow motion video.

Other benefits of plasma lighting include lower heat generation, compared to HMI and tungsten lamps; and full-spectrum daylight.

Because plasma bulbs are electrodeless and filament free, they are highly durable, maintaining high output and colour quality for some 30,000 hours of operation, and offering 50,000 hours of light.

The Future of Lighting

These different forms of lighting are competing in the continually developing marketplace of video.

Each has its own advantages, such as LED lighting’s efficiency and environmental credentials, or plasma lighting’s superior flicker-free qualities.

In terms of sheer performance, for example, plasma can provide excellent conditions for even the most demanding of slow motion sequences.

But then the alternatives offer their own takes on versatility, economy and durability, such as the lens-free M Series.

Ultimately, the future of lighting will be built on more choice and continuing improving lighting performance.

Whichever lighting method you apply, slow motion video will continue to be a powerful, visual tool.

It can bring a cinematic and artistic quality to video output; it can add emphasis, or increase an audience’s sense of anticipation. Slo-mo is highly effective in focusing the viewer’s attention on the narrative.

But to do any or all of these things, slow motion must look good, and this requires the technical tools to support the artistic vision.

Lighting is one of these tools.