Hereford Cathedral, UK. Lighting design by Light Perceptions.
Luminaires by Mike Stoane Lighting, Radiant, and Great British Lighting.
Directional light sources by Xicato. Cathedral photography by Audrey Kirk.

What's Inside

  • Hereford Cathedral - in 360˚
  • Introducing the Xicato Intelligent Driver (XID)
  • Understanding Fidelity

 

Click on the photo for a self-guided 360˚ tour of the newly illuminated Hereford Cathedral. Notice the different viewpoints, accessible from the rectangles in the lower right corner of the interactive screen. Have fun! And read on...

Bruce Kirk talks about Hereford Cathedral

Medieval Heritage

Hereford has been the center of a diocese since the 670s, and the site of Hereford Cathedral has been a place of worship since at least the 8th century. The first church was destroyed by Welsh and Irish forces in 1056. The cathedral that stands in Hereford today still contains remnants of the original cathedral built in Norman times (1079), and contains examples of architectural excellence from the late twelfth century to the present day, including the restored shrine of St Thomas of Hereford. It is a busy pilgrimage destination as well as hosting many cultural events, including every three years the Three Choirs Festival (a three hundred year old musical meeting of the choirs of Worcester, Gloucester and Hereford cathedrals.) 

In 2013 the Cathedral’s Precentor, The Reverend Canon Andrew Piper, and the Cathedral Architect, Robert Kilgour, concluded that the cathedral’s lighting was outdated in terms of energy efficiency, and was not providing the best possible environment for the many activities conducted throughout the year. They turned to lighting design and project management practice “Light Perceptions”, which specializes in heritage sites, to come up with a completely new interior lighting scheme, made possible by a £1m donation from a private family foundation through Hereford Cathedral Perpetual Trust.

We have asked Bruce Kirk, Managing Director at Light Perceptions, to write a guest article for our Newsletter on his design, in which Xicato light sources played the major role:

Before: sodium lighting in the nave

Before: sodium floodlights cast an industrial hue over the body of the nave.

Lighting a busy venue 

The relighting of one of the country’s major cathedrals can be a daunting challenge; these buildings are in constant use with its doors open to all comers every day of the year. There is a regular pattern of services, (perhaps three or four on weekdays), as well as civic and national services, school and university events, funerals and memorial services – all of which carry on throughout almost the entire lifespan of the project. There has to be a clear understanding of the lighting needs and usage of the entire space, as well as the smaller chapels in 360 degrees and from the often quite divergent viewpoints of the casual visitor, devoted pilgrim, musician, verger, tour guide and clergy member. Hereford Cathedral provides for intimate services for two people, and concerts for a full capacity of 1600. At every turn there is another variable!

From Fire to LEDs

Hereford Cathedral is a building with Saxon origins, built centuries before electric lighting was conceived. Candles and oil lamps were the main sources of light for the first 800 years of its life! Designing an electrical lighting system in a heritage building is a complex process, which not only has to gain approval from the Cathedral’s Dean and Chapter (equivalent to the Board of Directors) but also the Fabric Advisory Committee (locally independent monitors and guardians of the building) and nationally from the Cathedral’s Fabric Commission for England who have ultimate jurisdiction. A good working relationship with the Cathedral’s Inspecting Architect is essential.

Before: mercury floods in the Lady Chapel

Before: halogen floods in the Lady Chapel

Hereford Cathedral Detail: repetition in light and architecture

Detail: repetition in light and architecture

Goals and Plans

The solution for Hereford is led by its architecture; for the most part with a large number of smaller luminaires being positioned in sympathy with the rhythm created by the columns, arches and windows. Light must flow through all areas to provide a sense of connectedness, conversely it must also be able to be localised to create the intimacy necessary for smaller services in a number of areas. Continuity and consistency of design is important and whilst pendants and wall lights are used in certain areas to create localised focus, it is this flow of light that links the scheme together and harmonises the space.

The flexibility of the control system allows the vergers to set the scene for every type of use at the touch of a button. There are “global” settings for bright and dull days, services and major events across the whole cathedral whilst localised scenes allow for the subtle difference between Morning Prayer and celebration of the Eucharist in the smallest of the chapels.

Choosing the light source

At Hereford the advanced technology of LED light sources and the sophistication of the controls allows a lighting design created to address the complex demands of this fabulous building to function all day every day, subtly enhancing the experience of the visitor.

The main light source used is the Xicato spot module, chosen for its colour rendering and sustained colour point – as mentioned above, consistency is a thread that runs through the whole scheme. With this in mind, I have been successfully using Xicato modules in my heritage projects since 2010, including in The National Maritime Museum in Greenwich and the lighting of the famous Rubens ceiling at The Banqueting House in Whitehall.

Xicato Thin Modules (XTM) are available in lumen packages from 700lm to 5000lm, in 19mm and 9mm LES, in Artist, Vibrant, Beauty, Designer and Standard Series spectral formulas, and in 2700K to 4000K CCT.

Choosing the luminaires

The new luminaires, of which there are over 700, are more discrete than those previously used, and have both increased longevity and decreased energy consumption fivefold. Some were installed in existing positions, and some in locations not used before. In the transepts and certain side chapels, bespoke pendant designs used as a starting point the famous Skidmore originals (Skidmore was a famous Victorian metal worker), as well as incorporating parallels with the period architecture as detailed by Robert Kilgour. Flexible task lighting, lighting to display architectural detailing, and lighting for liturgical and theatrical emphasis are all incorporated. A paramount design consideration was the aesthetic appearance of the luminaires, which are clearly visible during daylight hours and have to fit comfortably into their architectural context. Another consideration of course was maintenance, with some of the spots being mounted 30m in the air. For example, luminaires in the tower gallery were specially designed by Mike Stoane Lighting such that focusing could take place from the tower walkway behind. For the same reason, where possible, gear trays were separated and mounted in accessible places.  Luminaire manufacturers included Mike Stoane Lighting, Radiant, and Great British Lighting, who made the chandeliers and decorated them with gold leaf. The contractor was R.T. Harris and Sons Ltd., who in the process re-wired the entire building, ensuring minimal intervention in its physical fabric: a challenge in itself.

The work was completed by the end of 2016 and well received. The Reverend Canon Andrew Piper commented on the lighting design...  "growing out of the daily life of the cathedral and serving equally well its diverse functions. It was also considered how best to celebrate the architecture, highlighting its rhythms and exhibiting its individuality, so that the design might enhance our appreciation of the space, as well as work in sympathy with the serene spirit of this ancient house of prayer. There is no doubt that the imaginative design has contributed a new sense of poise, beauty and mystery to the space which delights the eye and enhances the spiritual atmosphere of the building, making it even more inviting to pilgrim and visitor alike.”

Light and Architecture at Hereford Cathedral

Lighting highlights and accentuates the details of the architecture, while providing continuity through repetition.

Looking up in the Hereford Cathedral nave

If you enjoy looking at the beautiful work of our Lighting Design and Luminaire Manufacturer partners, you will probably be interested in the Xicato Galleries. Click on one of the buttons below to see outstanding projects, or to find luminaires for your own project.

Product News: Introducing the Xicato Intelligent Driver (XID)

Xicato quality dimming with Bluetooth control for a wide variety of LED light sources.
 
 

Just 46mm in diameter and 20mm thick, the Xicato Intelligent Driver (XID) comes in 1400mA and 700mA versions.

For lighting manufacturers and designers who want a single, open standard, wireless control solution for a wide variety of LED light sources, Xicato introduces the Xicato intelligent drive controller (XID). XID provides worry-free, hassle-free and economical Bluetooth lighting control with no compromise to power or performance.

XID are compact, programmable, deep dimming drive controllers that enable Bluetooth and/or wired control of most constant current LED light sources up to 56W. This includes the entire Xicato XTM portfolio, but also most 3rd party vendor LEDs, CoBs, and LED arrays. 

There are two XID models, which can be manufacturer-programmed using the Xicato Configuration Tool (XCT) to fit specific light sources and luminaires. One has a programmable maximum current of 350mA to 700mA, and another has a programmable maximum current of 700mA to 1400mA. Both accept either 24V or 48V DC input, provide smooth, IEEE 1789-compliant dimming performance to 0.1%, can be programmed with beacons, and provide detailed operating data over Bluetooth or DALI. 

Fast, Easy, and Affordable 

Xicato Bluetooth saves time and money, enabling simple, flexible system design. Installation and commissioning in hours or days, rather than weeks or months. No more expensive wires, hubs, or dedicated control gear. All without sacrificing fast, responsive performance or powerful features such as groups, scenes, schedules and sensor control. 

Industry-leading Performance 

Bluetooth architecture features direct, peer-to-peer communication between nodes, ensuring nearly instantaneous response to input from switches, sensors, or mobile devices. No need to hopto a hub or a central controller. Lights are intelligent; programmed to respond directly and individually to one or more inputs. 

Unlimited Scalability for Large Installations 

Xicato Bluetooth is highly scalable to meet the needs of large commercial sites. There is no practical limit on the physical size or number of nodes in a network. Tens of thousands of nodes can share a single secure network, and there can be tens of thousands of secure networks. Define thousands of groups or scenes, if you like. Extend the network using the Xicato Intelligent Gateway, and eventually through Bluetooth Mesh. 

Fully Automated Lighting 

Lights can be individually programmed to respond to manual commands or time of day, but the power of the system is fully revealed when switches and schedules are combined with occupancy (motion) and ambient light (lux) sensors. Put the light you need, where you need it, and when you need it. Automation provides the ability to minimize energy consumption to meet government regulations (and operating budgets!), but also creates a comfortable, convenient environment for people. 

Inherent Reliability 

Don’t worry the lights know what to do! The elimination of wiring, connectors, hubs andcentral controllers eliminates the possibility that one of them will fail, taking down the entire system. Distributing intelligence to the lights means that there is no single point of failure a failed node affects only that node. 

Beacons for Location-Based Services 

Bluetooth enables location based information and navigation services via integrated, programmable Eddystone, AltBeacon, and iBeacon functionality. Beacons act as indoor GPS satellites, periodically broadcasting tiny messages that can be read by mobile apps to determine the userslocation and provide relevant displays or wayfinding. 

Operational Management 

Xicato lighting nodes protect themselves by detecting thermal overload, and automatically reduce intensity to stay cool. 

Xicato nodes also collect and report data such as total operating hours, temperature, intensity, wattage and input voltage that allow facility managers to see and correct problems before they cause failures. 

Standards Based Interoperability                                                                              

Xicato Bluetooth is compatible with the technology built into billions of devices worldwide. Bluetooth has an ideal combination of features for lighting control, and is widely recognized as the likely foundation for the emerging Internet of Things. 

As a member of the Bluetooth Special Interest Group (SIG) and contributing member of the Bluetooth Mesh Working Group, Xicato is at the forefront of Bluetooth evolution, and is dedicated to incorporating Mesh compliance into its products as soon as possible. 

Tech Talk: Understanding Fidelity

What is Color Fidelity? How do we understand CRI and the new TM30 fidelity metrics and graphics?

Fidelity Myths

Many lighting people harbor misconceptions about the meaning of fidelity metrics such as CIE CRI Ra and TM30 Rf. For example:

  • MYTH #1: two light sources of the same CRI will look pretty much the same
  • MYTH #2: a light source with a higher CRI number will always look better than one with a lower CRI number

In other words, they believe they can directly compare two light sources by looking only at a single CRI score. This is not at all true.

Fidelity actually means Same-ness

There are two widely used standards for measuring color rendering fidelity:

  1. CIE Color Rendering Index (CRI), the older standard for which Ra is the fidelity index
  2. IES TM-30-2015, a new standard for which Rf is the fidelity index

Both tests use the same spectral power distribution (SPD) data to calculate how a tested light source would compare to a reference light source in rendering the colors of test samples. The maximum possible score in both tests is 100, which would mean that the tested source renders the colors of the test samples in exactly the same degree and proportion as the reference light source. There are two important things to understand about this: (1) it only compares the rendering of the test color evaluation samples (CES), and (2) it contains the implicit assumption that the reference light source is an ideal source.

The same as What?

The reference source for a CRI test – at least for color temperatures up to 5000K – is a theoretical black body radiator (BBR) of the same correlated color temperature (CCT) as the test source. In the real world, the closest thing to a theoretical BBR is a tungsten incandescent or halogen lamp, but even they are not quite perfect BBRs, due to the chemical elements that are being heated.

A BBR is not necessarily ideal, or even natural. For example, the Sun is not a perfect BBR. Nor is a wood fire. In fact, sunlight replaces the BBR as the reference for higher CCT sources, which creates an awkward discontinuity in the standard CRI test.

Furthermore, there are situations when you may not want a perfect BBR. For example, the Xicato Beauty Series was developed for a major cosmetics retailer who wanted very high color rendering fidelity for skin tone comparisons, but has a side effect of making people look and feel better.  Another goal may be to facilitate the observation of skin for medical diagnosis, as measured by the Cyanosis Observation Index (COI). Fortunately, you do not need to compromise traditional fidelity metrics to achieve an excellent COI score. For example, the Xicato Artist 3500K and 4000K, with CRI of 98, perform extremely well on this metric.

Reference curves for TM30 and CRI

Black Body Radiator reference curves for CIE CRI and IES TM-30-2015. CRI has an abrupt discontinuity at 5000K. TM30 smooths the reference curve between 4500K and 5500K to mitigate problems with color match at points just above and below 5000K.

The Problem with Swatches

A bigger problem is with the test itself. Traditional CRI as defined by CIE only tests 8 color samples (see below), none of which test highly saturated colors, and which collectively fall far short of testing the entire gamut of visual light. In other words, even a CRI score of 100 only means that a light source is exactly like the BBR on eight muted colors!

The list of swatches (R1 through R8) was extended to 14 or 15 by some, but these are rarely incorporated into the CRI number. More commonly, R9 has emerged as an important swatch that begins to fill the gap by providing a metric for red color rendering.

 

CIE CRI color swatches

CIE CRI color swatches

CRI was okay for incandescent and to a lesser extent HID and fluorescent sources, but it failed badly when comparing LEDs. After literally decades of debate, the IES finally took the bull by the horns and created TM-30-2015, which uses 99 Color Evaluation Samples (CES) (below) specifically chosen from real objects and dyes to represent a full, balanced gamut of colors, and which made every LED manufacturer's phosphor and die bin choice relevant to the score.

(Note: The CIE has adopted the TM-30 fidelity metric with minor revisions, and also calls it Rf, but they have not yet retired the old Ra metric, so the potential for confusion remains.)

 

TM-30-2015 color evaluation samples (CES)

Less than 100 Loses Meaning

What do we know about two sources that have a fidelity score below 100? Unfortunately, the further from 100 we get, the less we know. That is because the metric only tells us the average differences of the various CES from the reference, without telling us specifically how each color differs.

For example, a score of 85 means that, on average, the samples are 15 units different from the reference. All of the colors may be 15 units more saturated, or less saturated, or color shifted to one side or the other. More likely, a few of the colors may be very accurate, and a few others very inaccurate; some saturated, others unsaturated, others shifted. But we dont know which color is which! One 85 CRI light source may show highly saturated colors, while another shows drab, gray colors. And a source with a CRI of 70 might provide a much more pleasing effect than one with a score of 85!

Iso-Fidelity

This graphic shows that the same Fidelity number can be acquired by any type of deviation from the reference color rendering point. All points on the dotted line have the same fidelity score.

For example, the following two light sources have the same CRI (80) and TM30 Rf (78). Yet one looks entirely different than the other, and both are very different from their appearance under a reference light source. The source on the right is highly red saturated, while the one of the left is obviously undersaturated. And both exhibit color shift. (Images courtesy of Randy Burkett Lighting Design). 

80 CRI undersaturated

Light source: CRI 80, Rf 78, under-saturated

80 CRI oversaturated

Light source: CRI 80, Rf 78, over-saturated

Providing Meaning

To help lighting designers choose, TM30 provides several informative graphics that present Fidelity in different ways. For example, the following graphic shows the fidelity of the Xicato Standard Series (83 CRI) in 3000K CCT. A taller column represents higher color fidelity in that particular color, with a maximum of 100. The average of all of these values yields the TM30 fidelity metric, Rf, of 78.

 

Xicato Standard Series fidelity by CES

Xicato Standard Series TM30 fidelity on all 99 color evaluation samples

A casual observer might assume that the relatively short columns in yellow mean that there is less yellow in the light source. But the reality is the opposite; the yellow in Standard is lower in fidelity because yellow is OVER saturated, as shown in the following graph, also from TM30, which shows the average saturation of different groups of colors.

Hue angle graph, Standard 3000K

Hue angle graph for Xicato Standard Series 3000K, showing color groups that are over- and under-saturated.

From this graphic, you can see that yellows, greens and purples are slightly over-saturated, while reds and cyans are relatively under-saturated.

Now, lets look at a very high fidelity light source; the Xicato Artist Series, which has a CRI of 98 and a TM30 Rf of 96 (TM30 is a tougher metric!):

TM30 color fidelity for Xicato Artist 3000K

Notice that all of the columns are very tall, with no significant gaps. In fact, many colors are spot on, and no color group deviates more than 3% in saturation from the reference, as shown by the graph on the right:

Hue angle graph for Xicato Artist 3000K

So far, we have talked about how the fidelity score indicates the average deviation of a test source from the reference on a selection of CES. We have shown graphs that illustrate variation in saturation, but what about color shift, which is typically a much more objectionable variation?

TM30 has a graphic for that, too... in many ways, the best single graphic for understanding a light source lit effect:

Xicato Standard Series 3000K color vector graph

IES TM-30-2015 color vector graphic for Xicato Standard Series 3000K

On the left is the TM30 color vector graph for the Xicato Standard Series 3000K LED light source... the same as shown above. Observe that there is a color space with a gray circle and a red, roughly elliptical shape superimposed on it. The gray circle represents the "ideal" color rendition of the 3000K reference source, and the red line represents the relative color rendition in each of the 16 hue bins of the tested source.

A perfect tested source would of course be superimposed directly on top of the gray circle. Like the fidelity graph above, this graphic shows you again that yellow-greens and purples are slightly oversaturated in the Standard Series product, and that blues and reds are slightly under-saturated. But it tells you even more...

Notice the tiny arrows that point in or out from the gray circle toward the red figure. The length of these arrows tells you the degree of variation from the reference, and the direction tells you in what way the hue bin differs. 

  • Oversaturated = radially outward
  • Undersaturad = radially inward
  • Hue shifted = tangential

So as is typical for most light sources, Standard is slightly hue-shifted in areas near the intersection of the two lines.

Now let's look at Xicato Artist Series...

IES TM-30-2015 Color vector graphic for Xicato Artist Series 3000K

This is a larger graphic because otherwise it would be very difficult to see the tiny arrows. This is a good thing! What you see is that the red Artist line almost perfectly overlays the reference line, that the arrows are very short, and that they are all radial. In fact, Artist is ever so slightly oversaturated where it deviates, which is generally preferred, resulting in a slightly larger color gamut than the reference source.

This is why Xicato Artist Series is an ideal replacement for halogen lighting. The high fidelity, balanced color rendering of Artist Series has made it the preferred light source for art museums worldwide. It is used by high quality retailers and even grocers for its ability to reveal product, bringing out the rich colors of wood, ceramics, fabrics and leather, as well as the true colors of produce and packaging.

Conclusion

You can see that simplistic fidelity metrics that present a single number – especially as the number drops further below 100 – do not provide complete information about the lit effect of a light source. Of course, the best meter is your eyes, and the best test is to compare actual sources by shining them on the objects that will actually be in your space. Absent that, TM30 is a much better tool than CIE CRI, providing much more information that enables lighting designers to make better choices for their clients.

Xicato was the first to publish full TM30 data on all of its color series, and we continue to be available to lighting designers, manufacturers and end users who want to learn more about color fidelity, gamut, quality, and metrics. Contact your Xicato representative or write to marketing@xicato.com for more information.

What can YOU do with Xicato Bluetooth lighting control?

More and more lighting designers, specifiers, and OEMs are getting familiar with wireless Bluetooth control, sensors and beacons using our XIM Gen4 Evaluation Kit. The XIM Gen4 development kit comes with two XIM Gen4 modules on heat sinks, with optics and power supply, a USB BLE dongle, and the Xicato Intelligent Sensor (XIS), which is a Bluetooth integrated sensor with passive infrared (PIR) motion sensing, lux sensing, temperature, humidity, and an accelerometer.

Making full use of the kit requires the Xicato Control Panel software for Windows, and the XIMtroller iOS software, both of which are available free of charge from Xicato and the Apple iTunes App Store. Click below for more information.